To be fair and within context...Bush was holding the dog with both hands and clumsily attempted to salute....well-intentioned, but incorrect form.
Obama's left hand was free, and could have transferred the coffee to the other hand, but made a limp salute, coffee-in-hand. I think people's "outrage" stems from external perception...and it's fair to state that Obama appeared to salute in a manner that screams "ok, fine...here's your damn salute".
We also have to remember that other presidents have paid dearly for perceived outward impressions. Bush during Katrina, is one example. No one knows how he really felt about the victims in New Orleans and elsewhere along the Gulf coast, but the way he came across on TV seemed full of ambivalence, and he paid for it in the public's opinion.
With the "latte salute", there's already a general perception among a lot of people that Obama doesn't really care for the military, and that salute only helps cement that image. Personally, as the CinC, he should at least learn to do the ceremonial stuff right. It's not like those Marine's ambush him at the foot of the stairs each time he steps on or off an aircraft...and his people should know there's always going to be a ton of cameras recording the event.