Quite a few reasons, and lj summarized them quite well.
A discussion works both ways, and when somebody blows off time tested facts as propaganda, that's not having a civil discussion. Calling time tested facts propaganda is an argument we hear from the left and often the young.
If this were in fact a bannable offense then almost every creationist in the evolution threads would be banned by now. If you want some examples I'll be glad to provide some.