The other problem with backup during that initial upheaval and/or possible bomb strikes that people are talking about is that you have to remember what was going on at that same time in the middle east. Cairo really was having protests about the video, and that at least partially seems to have inspired part of the mob in Libya. Libya is an incredibly armed country. Everyone has guns, and guns are everywhere, because they just got through a revolution. Now any mob, whether violent or not, is likely to have these, and this mob was indeed shouting at least partially about the video. This mob did turn violent, yes, and the ambassador seems to have hid at this time in a safe haven.
Now, it shouldn't take a genius to see why bombing, or doing any hasty action to a mob like this would be a very fucking bad idea. You could essentially ignite a war in the middle east with an action like that, because it could be seen as bombing people protesting over a video defaming their prophet. No matter how much you protest, that's how it would be seen to them.
On top of this, Libya is a fledgling democracy struggling and trying to get control into the government's hands. They need legitimacy, and the US has to help them gain it for their future, their citizen's future, and for our future. That's likely part of why the scuffle was left to the security team at the embassy. That's also, again, why you don't just bomb things right off the bat. Cooperation with the new government is key.