Who says they let him walk, UFC had matching rights, but who's to say that Bellator didnt structure the offer in such a way that UFC couldnt match it.
Bellator have in the past put in clauses in the contract that while costing them nothing basically block UFC being able to match.
Things like
+ We will give you 10% of the gate where the gate exceeds $5m
or
+ we will give you 20% of the ppv revenue when the PPV exceeds 500,000
Bellator can put in clauses like that that basically have no effect on Bellator as they are never going to trigger the clauses, but by including them that means that UFC have to match them.
They are basically put in by Coker to prevent UFC being able to match the contract.
If Rory wanted to go to Bellator, its very very easy for Coker to structure the contract in such a way that its impossible for the UFC to exercise matching rights.