Everybody squabbles over their soverign territory. If the Army were to drop a bomb on somebody, the Air Force would get their feelings hurt ('specially since they used to be one and the same). Kinda hard to always put the mission before the feelings, I guess.
Back in the 90's the Air Force was talking about retiring the A-10s. The Army said they would take over CAS missions and fly the A-10s. The AF went ape #@$^ over that one. I do not remember how many months the fight went on but the AF kept the A-10s so the Army would not get them. It was like two little kids fighting over a toy.
It would actually make sense, since the A10 is primarily a ground-attack aircraft anyway (Sidewinders are more an afterthought on the Warthog!), and generally its missions make life alot easier for the groundpounders. It would simplify things if the Army didn't have to ask the AF for a mission.