Yep. I think it goes something like this. Rebel Pirates >> the Marines == Seven Weapons of Sea.
It's a good deal for the Marines. You just have to forgive sins of few pirates. It saves some resources and strong pirates are killing each others. The only problem is incapability to control the 7 SeaWeapons but I bet there is another Bukai besides Mihawk that isn't evil. Jinbei the good guy would be silly
.