It isn't a rule, but the admins are trying to make it more fair for humans on escape maps. Though I can't say I agree with them, because I've proven to them that you can survive on most escape maps against fast zombies. I even proved it on train escape.
It's a lot like a common courtesy to not be a fast zombie on an escape map though, just as it is a common courtesy to ztele out of a barricade if your the first zombie. But I certainly hope they aren't banning people for it.
The owner of the servers will be able to modify what classes are allowed depending on what map we play to make it more fair for humans, but I wouldn't expect to see that for awhile. At least until the maker of Zombie Mod comes out with the next update.