It's human nature. I wouldn't and I don't think anyone would post stuff like XXX sucks and XXX is rubbish etc, but I do think people would post controversial things which naturally provoke folk and if they can't reply to the original thread they may reply by starting and locking their own post.
If I posted a post at why I though Tony Blair has made a brilliant job of being Prime Minister, loads of folk would be annoyed and would think him a criminal. If I posted he should be removed from office because of the Iraq war or that he is abusing his position and robbing people in tax - people would defend him to death.
Seeing as half of people here are American, US politics (which is a very volatile and disagreeable subject!
) would be even worse. It also doesn't matter how tactful you are (you either love politicans, you hate them or you are not interested in them) there will always be someone that will be upset.
It is simply only because of human nature that the no-politics, no-religion rule exists. It isn't because of the subject itself that it is not allowed.
See what I mean?