We generally allow honest liberals interested in fair debate to stay.
However, honest liberals anymore are more rare than consistent positions by John Kerry, so that's why you don't see too many of them here.
I think that's a fair statement. As long as these folks come in and at least try to answer what is asked of them (hard to do, but try) as opposed to being "troll" like, posting something like "Bush is a Nazi" in a new thread, then never replying to dissenting opnions or posts.
If DU truly bans you as soon as you say, "Hey, I'm conservative, and I think you're wrong for the following reasons...", then their policy is indeed different.