To me, it's first come, first serve. If another player/group is already at a camp, for whatever reason, when I get there, I'll find another spot. If I'm there first, and someone tries to camp on top of me, I'll yell at them, and do what I can to make their lives miserable.
But as someone above me said, there is no stated rule. An unclaimed mob is unclaimed. This is just a courtesy that has extended way back into XI.