GE was simply repeating a transaction that they had made for the 737-300.
They paid Boeing about $250 million with the proviso that Boeing would have to pay it back if that model was offered with another engine. So the entry fee for RR and Pratt to participate in the very successful 737 family growth became prohibitive.
Given the cut throat competition in the civil engine market, GE's decision to pay up front for exclusivity has proven to be a brilliant business strategy. They don't need to do 'strategic' (aka loss leader) deals in those markets anymore.