It is big time illegal to fire at any aircraft in the US just ask the FAA.
I know this may come as a shock to many here at FR, but we are actively helping the Government of Mexico fight this drug war the same way we helped Colombia. Earlier today Mexico arrested their number 1 heroin trafficker. The only reason he is under arrest is because CIA, NSA, DEA and who knows what other US agencies were able to track and pinpoint him.
Why on earth would we want to shoot down a Mexican helicopter that we paid for?
Good point. But do we not have choppers that could patrol our side of the river?