Tree houses are different.
They're breakable, and take a lot of time to make.
It's just like getting behind the fence in anchor.
jim told me long time ago what should i consider banable and what i shouldn't.
If i there is the phrase "out of the map", it is banable.
getting behind the fence in anchor got 2 advantages.. that just shotting a box only fast zombies can get you it is a secret that if none zombie knows is a auto win for humans.(also, almost all the fence got a invisible wall... what would make it kinda the end of the map)
but in checkpoint... getting on that place doesn't give you that good advantage since fast zombies can kill you quickly. (in checkpoint you're still inside the map and easy to get by fast zombies.)