Purely by speculation, I would guess that DC probably didn't want to pay the White House liaison agency, or whatever group Marvel paid money to in order to use the President's likeness, the fee they would need to. And probably wouldn't feel they would get it as they would put the office of the President in a less than flattering light (i.e. the White House being penetrated by a single rogue agent who can tell off the President and then walk away).
So, backlighting to show no definite features, and also note they never actually refer to Obama by name here. That way if the legal agencies call them out on it, DC can say "Hey, this isn't meant to be the President of the United States, this is the President of the U.S. in the DC Universe. He just happens to look a lot like the current President." And thus are shielded by parody/free speech/various legal grounds.