What do you expect? This is another drop of outrage in a sea of outrageous behavior, coming from a man who thrives on behaving outrageously. While not excusing him, I can say that I understand why it happened:
1) He's got a lot on his little twisted Stalinist mind right now! Dealing with an administration so rotten, festering and riddled with scandal, and just finding out about it on the news last week? Why, I'd be a little off my game, too!
2) He went back and shook the Marine's hand. I assume he apologized for not returning the salute, but I would have given him a complete pass if he would have returned the salute after apologizing. That shows respect and humility. The hand salute is a traditional gesture of respect and honor between warriors. Obama is not fit to have the label "warrior" associated with his name anyway, so even when he returns the salutes, I have always considered it an empty gesture.