Whether or not that classifies as "fair game" indeed remains subject to interpretation. To me, it looks like there's an exploitation of a bug (namely, the lack of latency control in PS), so from this PoV it can even be considered cheating. Be it as it may, as has been said, in camping one spawn there might be a justifiable claim of, if not ownership, but semi-entitlement. However, with more than that, there IMNSHO can't. Sure, it is nice to have a continous stream of MOBs coming in all the time, but there's no justification in claiming all of them. "Rude" is the nicest word that applies here.
Luckily that's not always the way things work. In fact, some time ago I was visiting the arena and merely watching two people camping a MOB, when I was asked by them if I'd want to share.
Oh, in case you're worried about the mute bug, /tell isn't ever affected by that (merely by packet loss in times of excessive lag, and message misrouting seems to have been resolved for some time by now).