You are making the assumption that the Mexican military is as well equipped, trained and has the same resources that a US pilot / crew would have. I can tell you for certain that is NOT the case. Since they were running drug interdiction, likely they thought they were firing at drug dealers.
They were 15 yards from a vehicle with 4 border agents inside and they fired at the vehicle.
The shooting, let alone the border crossing, was inexcusable and unprovoked.