Because as long as Bellator is willing to match what the UFC is willing to pay him he either has to sign with Bellator or wait out the matching period. I don't know why the UFC just doesn't do a contract Bellator can't match, give him a small PPV % for his first fight or something, Bellator has no PPV revenue so they could not match it. I am sure the UFC could use tricks to get the contract but I think they will have to and Bellator will lawyer up to fight it and the such. This kind of thing was common in Prowrestling when WWF/WCW were at war.
Sucks really, would love to see Alvarez in the UFC. Surely as you say the UFC could do something about it, no way they can't make a deal that Bellator can't match.