Again, business. Nothing is forcing the arenas to agree to enter into a "no compete" clause (except perhaps that they want WWE back in their buildings).
I don't see it as unfair at all. It simply shows how much clout the WWE has, and how much TNA lacks. Before anybody says it, it should not and does not matter to the WWE to allow TNA to gain the same kind of clout. If TNA can't do it on their own, then they lose. Plain and simple.