I think it's an age-old custom. There are two things that are off-topic for casual conversations with non-family-members - political and religious beliefs. People tend to tie them to their egos, and it pretty much always results in resentment. It's a general population equivalent of inexperienced developers arguing about merits of programming languages or text editors.

'pg actually wrote an essay about it once.

http://www.paulgraham.com/identity.html