There was many discussions about this before, it is allowed, besides it doesn't happens that often like it used to.
Whenver I'm on, and we're in Castle Age, infected hospital, abandoned facility, and maps with similar z-spawn layouts [even to a far less extent Checkpoint] [train terminal being a very infamous map so far as humans dicking zombies over
], people camp in the green fog at spawn, throw nades into the green fog, zombies die within seconds after they spawn and begin moving [without even having to move out of the green fog covered areas completely], if not as soon as they are able to move. Lots of text and voice chat complaints "get out of the spawn" from many players.
I dunno if it's just with the people I'm on, or if you don't see it being in other parts of said maps fighting zombies etc, but it is still very common in these maps [which come up frequently in voting] when I'm on.
Speaking strictly personally, I don't see why it should be allowed - killing as they exit a spawn room [short of blocking teleports if the z spawn is separate] is one thing, but going INTO where they spawn, and waiting for them to spawn, and then killing them as soon as they are able to move doesn't exactly make people WANT to play as Z. Case in point: it SEEMS to me anyways more common to find people whom dread starting as zombie for just the above issue I mentioned, especially on maps where humans can do this ad nauseum from the start.