Why does censorship exist?

I am constantly amazed by the degree to which people not only engage in, but cheerfully accept the practice of censorship. For example, I’ve often experienced wholesale (and often capricious and gratuitous) censorship on Internet fora where I contribute, and if I complain, the complaints, too, are usually censored—and no matter where I go, and what form of censorship occurs, there is a long, long line of people cheering and applauding the censorship and demanding more.

My question is, why does censorship exist? Given the degree to which people seem to enjoy silencing the views of others, I can only assume that it serves some practical function. And no matter how loathsome the behaviour, from rape to warfare, one inevitably finds an evolutionary advantage at the bottom of it. With censorship, the opposite seems to be the case.

Where censorship exists, there is a willful desire to hide oneself from all unorthodox views—and further, to hide the tribe from such views as well. Yet the survival advantage seems to be in exposure to as much information as possible. Tribes which refuse to listen to “heresy” risk losing access to important information. During a period of environmental stress, a tribe which engages in censorship seems to be less likely to survive, thus making genes and memes which predispose to the behaviour self-eliminating. For example, suppose Tribe A censors all information regarding The Holy Place where the gods are said to reside. Tribe B does not. When famine rolls around, Tribe B will have that little bit of extra knowledge about hunting grounds or plant resources than Tribe A and will be more likely to survive.

Given the degree to which censorship exists (and has existed in every culture of which we know), we must assume that it is an instinctive behaviour. I can’t seem to find any evolutionary advantage to punitive orthodoxy. Can anyone else propose a theory to account for it?