what is the theory behind maple stands and racks?

Shadorne, and they quite believe they are right!

Maple is easy to look at, reasonably stable, easy to source, easy to machine, and easy to finish. That's it, nothing more. Well, except that in some uses people believe it sounds better.

I built my stands using maple and walnut. Why? Because both woods are easy to look at, reasonably stable, easy to source, easy to machine, and easy to finish. ;-)

In my system I found that maple shelves (all of my shelves are on sandboxes) work ok under most of my components. It was horrible, IMO, under my turntable. So I tried granite. it was horrible in another way. I eventually wound up with 1/2" aluminum plate under my 'table, the table is spiked into the plate, and the plate is supported above the granite shelf with Stillpoints. I'm using rollerblocks under my source power supplies (that's where the tranny vibes are), and I let the stock rubber booties handle the CDP, preamp and phonostage chassis. So, in the end, I'm finding that no one shelf material works for me in all applications.

But if you notice I'm using several forms of vibration control. Some isolation, some damping, some coupling. It all just happens to be sitting on those maple and walnut stands.

Maple isn't really all that hard, IMO. The vibrations that enter the platform are bounced around and some converted to heat within the wood. With most butcher block platforms there are many surface boundaries which effect the waves traveling through the wood. This causes even more scattering, but there is also attenuation as some of the energy is lost in the reflected wave. Eventually, things settle down.

The butcher block I used caused a smearing of the midrange that resulted in loss of detail and too much warmth. I may try a solid chunk sometime, but I suspect it will still be too warm for my preference.

There are many folks using big hunks of maple as shelves and I'm sure it does work in many applications. But obtaining better sound by using maple is no guarantee.

And those spikes that folks think act like some kind of mechanical diode, well it doesn't work that way. The spikes provide coupling which means vibrations can travel in both directions. They can be effective in some cases and detrimental in others. In general, spikes help emphasize bass and devices like rollerballs (non-compliant) help emphasize air and space around instruments. Products like Stillpoints are somewhere in between as they have both high-rigidity and some compliance. Personally, I have better luck getting the sound I want by using these kinds of products over what material the shelf is made from. YMMV, and all that.