As you said, developers in these sorts of communities will often be plagued with the 'idiot' type.  You can't really compare this to bus drivers, for example, as I'd say that the majority of people the bus driver interacts with are probably nice.  Whereas you probably get exposed to a large majority of "not-nice" people if you decide to give support in these sorts of communities as a developer.
Even if they know that those interacting with them are the "bad majority", it's quite difficult to really ignore the fact that you are constantly dealing with these, and after a while, I guess at some point, you'll snap and decide to just simply leave.
And getting your code 'borrowed' can really hurt some people.  Some of these things are intellectually challenging and take hundreds of hours to make - someone taking that away can really get one worked up.

On the other hand, I've seen idiots get all protective over a 2 line script.  Go figure :P