RE: the question: ":Why do forum interactions become contentious". . .

Indeed. I cant imagine getting so worked up that a nasty PM would be an option.

When people don't have a lot going on in their lives  they tend to focus on tiny minuscule things in life.   A lot of the strife in the country right now is caused because people have too much time on their hands due to many factors that you all know about.

Once we're "allowed" to interact more in person, a lot of this heat will die down (at least I hope it will)

I strongly disagree with those who are saying that people who make these nasty comments online also make them in face to face conversations. People who make these comments in online forums are hiding behind the anonymity that online forums provide and are likely much nicer in person because they realize that if they said some of these things to someone face to face, it wouldn't be long before no one would associate with them. For that reason, I now refer to all social media as antisocial media because that's what it has become. 
Are there appropriate moments when a good putdown is needed for a person you will never associate with again in a face to face situation? Darn right there are, but they are very rare and you have to be creative and use just the right putdown to be effective. Calling someone an idiot is uncreative, lazy, and worthless, so the OP can take away from this that the person who wrote that reply to him is uncreative, lazy, and worthless as well.
BTW, MC brings much of the vitriol on himself by projecting an unjustified overconfidence that only he can be correct. That's hogwash and we all know it.

Some folks get so emotionally invested in their opinion that they perceive challenges to their opinion as somehow challenging/insulting themselves.  I suspect many anti-cable folks fall into this category and are so emotionally entrenched that they cannot emotionally look at evidence to the contrary.  Many of these folks experience emotional pain when they admit they’re wrong so avoid it at all costs.  The truth is that varying opinions leads to debates which help us to learn and grow.

Always look at the bright side. It beats shooting each other. 

It’s a great philosophy.  An attitude of gratitude leads to a joyful life.  Too many people focus on what’s wrong with the world which isn’t healthy mentality.  If one focuses on the news, which is heavily slanted to the negative and entertainment ratings, will lead to a negatively balanced point of view.
The good news is that people can change for the better.  How?  Thinking is nothing more than asking and answering questions. Negative people are constantly asking themselves “what’s wrong with ___”.  So asking better questions such as “ what’s great about this, what can I learn from this, what I am grateful for” creates healthy thinking.  Also, intentionally changing focus from a negative to a positive also greatly helps one’s mentally.