Plot twist and a theory:

Think about this. he's not the legitimate president. he wasn't allowed to talk IN independence hall or go inside. That's why he spoke in FRONT of it. The marines were real, and were there guarding the entrance to make sure he doesn't set foot inside since he's not allowed. That's why there is the fake oval office, the fake president's desk, and his "my fellow americans" speeches are all from different parts except INSIDE the oval office and such. That's why he talks to the press in weird places. That's why he's not allowed to go in certain "sacred doors" or passages. and on and on and on. that's why something doesnt feel right about any of this. Because it's not legit...

Many in the military know he's illegitimate and doesn't have authorization to do most of these things. Therefore, the marines you see around him aren't guarding HIM, they are guarding the places and things around him to remind him he is NOT the legitimate president.