This seems like something that the devs intend to do but, as people have said, haven't yet because of the obvious problems with zombie behavior and walls. There's absolutely no reason to not have them running indoors once they won't be glitching through walls and such. Yes it makes the game easier, but I think Rocket has made it abundantly clear that that alone is not a justification for doing anything. Making them walk indoors would just be an arbitrary decision to make the game easier, which doesn't seem likely. That said, there's no reason to make the game impossible. Until zombie behavior is fixed (which it will be- Alpha!) there's no reason to think about this stuff yet.