Every US military installation in the world displays the current upper echelon of the chain-of-command in their offices, buildings, and headquarters, and I can assure you that every US base in the world presently shows President Obama on top of that command. The POTUS is the Commander-in-Chief, and for a Marine to insult the POTUS is just as insubordinate as insulting the Commandant of the Marines (technically it's worse). Furthermore, every active duty soldier is told to keep their political opinions to themselves - it's common knowledge in the military, and this soldier clearly thought those rules didn't apply to him. The military handles soldiers who don't follow rules in a very predictable fashion...
Tough titties, Sergeant.