I don't think that PC is the primary reason.
But, it's easy to write something defamatory but difficult to prove it, and what next?
Usually, any legal charge is weighed with all evidence by a jury, and it's considered wrong until proven right. Who's the jury here?
Posting "A is a (blabla)" should be considered wrong (= a lie) until an independent verdict, but instead it will be spread - as a "lie" - across borders, multiplied, magnified without control. And, nobody can take it back.
Second, personal arguments produce strife, like Religion and Politics. None of these are neccessary here, and people have been harmed from such things, so some subjects better stay outlawed.
Read the Posting Guidelines and follow them - they are good for all of us!