Because he won a Nobel peace prize? It's good to remind people that he isn't a good person. Everyone likes to pretend he was "the only good president" or something. He did evil things too. Like "crimes against humanity" evil.

The cartoon works because it plays on people's preconceived notions of Obama.

Also it's possible the cartoon was drawn while he was president, in which case he would represent the US just as well as an American flag for purposes of a political cartoon.