President is a different thing, I'm pretty sure that the President is the only "civilian" in the military leadership chain as the "Commander in Chief" so there is an expectation that they'd return a salute. The requirement or expectation to salute is kinda wobbly when there is something in your hand or carrying something in both hands or if your doing something.

For what it's worth, the expectation is that the subordinate will salute more than the ordinate will return the salute. But it is pretty rude to leave somebody hanging like that so Obama getting some criticism was more justified.