It may allow American forces to (more easily?) go down there and run their own operations, potentially refusing to cooperate with Mexican authorities and potentially messing up their own plans and just overall making things muddier and trickier, especially if they rely on sting operations that work better with fewer things going on to provide suspicion or otherwise serve as confounding factors.

With that said, I would imagine that cartels are international terrorist orgs, so I’m not necessarily arguing for or against one side; I’m just trying to imagine why Mexican gov might dislike it.