Why do people say low power amps should be good for bookshelf speakers?

I was in a BB Magnolia recently and they had a McIntosh MHA150 integrated headphone amp that can also do 50 watts per channel to speakers. The sales rep said it "should be good for small bookshelves but its not enough power for towers". 
I've never understood this line of reasoning.  Towers are typically more sensitive than bookshelves. Is there an actual reason a small amp like this couldn't do just fine for towers that are equally or more sensitive than similar bookshelves?
Very ignorant salesperson. Many bookshelf speakers are power hogs. He should know that.
Even bookshelf speakers need power. 
  Run a old BIC d62  pair with a yammy receiver w a tiny 80 W RMS. 
      RAN THEM WITH MY SUNFIRE SIG 600, No I didn’t  kill them, I played them with their power as specs, sounded great, 

    Know the limit of the speakers, and how hard to push them.   Have 3 systems, main, 2nd and my 3rd which is BIC v-630’s with yammy 80 W receiver , hooked to my SL-1200MK2 to record to computer, with a few cartridges, Arkiv, and nightclub II TO RECORD LPS TO HARD DRIVE 
 USE audiolab 2.0.
  Then record to cd to play.

 I love the snaps and pops of the recordings!j

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I remember Platinum Audio bookshelf speakers. Real power hogs. Needed about 200 wpc to get them to sound right. 
Is speaker sensitivity the bottom line for determining which amp is sufficient? Or do the number of drivers in a tower become a factor? Do more drivers require a bigger amp for sufficient control?
Yeah don’t listen to Best Buy employees. 99.9% of the time the larger speakers are easier to drive as they have bigger drivers than need to move less for the same sound pressure. 
Hi Robert -

It is a combination of factors ... right?  

1) Speaker sensitivity for sure.  2) The amplifier's continuous power output per channel measured across the full frequency range (WPC, 20 - 20,000 hz).  3) The size of your listening space.  4) How loud you like to listen or better said, are allowed to listen (apartment house vs private house, family complaints).  5) Minimum vs Maximum Power.  You should have more amplifier power than you really need as you don't want to max out or floor your amp.  Need clean power.   

It is not a tower vs bookshelf thing, per se.  It's how all these factors come together.  I assume that this is for music and not home theater, as HT really eats up power.  

Sales rep did not know his stuff or is compensated to direct sales in a certain way.

Best, Rich 

"People" say all kinds of goofy things like 
"The earth is flat". 
Questioning their reasons is futile and   
Frankly I don't care.
The amp and speakers are compatible and will make sound and he wants to sell stuff so put 2+2 together. That could be the extent of his expertise.  Most customers there would never think twice about it. 
It is either ignorance or the salesperson pushing a specific sale or a simplification of what is actually a little bit more complicated.  Smaller speakers might tend to be more inefficient and they also tend to be not as capable of delivering very high volume level.  A lower powered amp will reinforce the notion that they system should not be played as loudly as one might play a bigger speaker.  I know it is a common belief that more speakers are destroyed by pushing a lower powered amp into clipping, but, ask most speaker manufacturers and they will tell you that is not the case--most speakers are damaged by simply being overdriven (even if the signal is clean). 
Definitely an ignorant comment or as Larry says a salesperson with an agenda. And my eyes must be playing tricks I thought I saw a post by @viridian talk about pleasant surprises!
I've seen a number of online reviews in which the reviewer states something like "the amp worked well on my small monitors, but it probably wouldn't have enough power for large tower speakers."  Total ignorance of the basics of speaker sensitivity.   
The only reason I can think of for saying low power amps work well with bookshelf speakers is low power amps tend to be small and light enough to fit on the bookshelf.
The other reason could be that bookshelf speakers tend to be used in a lower volume type of setting.
1. Some salespeople will say anything to make a sale.
2. Some salespeople are totally ignorant of the products they sell.
3. Some salespeople are alien agents looking to weaken our social network.
Take your pick.

3. Some salespeople are alien agents looking to weaken our social network.
Take your pick.
Now thats funny.

+1 noromances.  I too think the salesman was making a generalized assumption that smaller bookshelf speakers would be listened to in close proximity and at lower volume levels and would thusly require less power. In some applications, but certainly not all, he would be right.
My Monitor Audio Gold 100’s are power hungry 4 ohm bookshelf speakers with 86 db sensitivity rating.
The only reason might be to eliminate over exuberant use of volume, which could damage speakers, so there can be no come back on the retailer.

Maximum power is often quoted when THD+N reaches 1%. In my mind that's a dreadful figure.

Therefore maximum power is probably best considered at half the advertised figure. Just don't screw the volume up to full.
You are correct there are a lot of big speakers that would thrive on fifty watts per channel and there are a lot of bookshelf speakers that would too because they do not handle much power but everything is about mating the right amp with the right speaker so it is best to try the amp with the speaker that you like and see how the combination works in your room with your music with the rest of your system.
I've basically got the same amp, the MHA100 and it's a REALLY NICE sounding set up through the speaker taps. I'd say 90db or better for a larger room or loud volumes....bookshelf or tower. My 95db bookshelf speakers never make the amp break over 2 watts even at spirited volumes. Enjoy. 
Salesperson speaking here...Noromance & aewarren are on the right track.  The BB SP was making an assertion that contained an assumption, which is not great critical thinking, but the gist of it is that people who want big sound in big rooms should buy big speakers and amps to match...and the converse is also true.
It depends.  Many bookshelf speakers are not sensitive but also don't have deep bass response.  When SPLs go up to high levels (and power draw goes high), usually it is at the lowest frequencies (below 40hz).  You will rarely see frequencies above 60hz rise more than 10dB above average volume levels even in the most dynamic music.  

I have speakers here with a frequency response that begins rolling off at ~60hz and 45w is plenty to drive them at reference levels despite having an SPL of 84dB.  If they started rolling off at 40hz, I would be far more concerned and would seek a lot more power.   
That is too broad a statement 
my Elac Vela 403 speakers are 87 db efficient and are a 4 ohm load  they need a little extra  power to get them working optimal
.the-true  efficiency of speaker,not just rated  ,many companies fudge the actual  efficiency. Unfortunately there is no standard how to test from true 20 khz to 20 Hz sweep for a specific time and spl level would be good if all companies did this, and  on bass at least go down to advertised specs,for a extended period ,and how easy a load is the Xover ? these are 2 key factors  how a amplifier will respond . Sometimes getting  the synergy is a lot of trial and error .
I have found many of the Magnolia sales people to not know what they are talking about.  This is why I would prefer to talk to a good independent audio dealer.
people say stupid stuff because either they are stupid or they think you are
A lot of good advice and logical reasoning above. I use Kef's relatively inefficient LS50s in my desktop and bedroom rigs. The desktop pair is driven by a low powered amp that is more than sufficient due to my proximity. 

I'm about 16 feet away from my bedroom LS50s in my usual listening position, and initially drove them with a 50 wpc Marantz receiver. The sound was fine for late night listening but fell apart if I cranked it up. I recently upgraded to a Purfi based amp and the LS50s really came alive. 
I don’t know why, I have a 2 x 400 Watts potential driving my stand mounts??

I have read very good things on that Purifi class D module, and their 6.5" drivers