That's the reason for sure. Someone will see kids flying planes into buildings and freak out and then Rockstar gets sued.
thats a pretty silly reason tho. theres alot better games out there that can do THAT better (such as microsoft flight sim), because rockstar really cant make the planes in their GTA games to fly as they should.
besides, they should have thought of that before releasing San Andreas, tho with the crap flight controller of that game u might want them to use that, they will crash for sure then before hitting their target.
owell, there will be mods out eventually that adds flyable planes for sure, i'm possitive of that.