I think that an even bigger etiquette question than whether or not to use a flash is whether or not the photographer is impeding people who have to go by, or whether or not bypassers are interfering with the picture.
For example, if this was a pre-arranged shoot with the flash, say by a professional photographer for commercial purposes, I think it would be really rude of bypassers to interfere with that. They could do damage to the tune of hundreds, maybe thousands, of the appropriate currency to the photographer, the subject, and the party who commissioned the photo shoot.
On the other hand, I'll agree that someone who acts like a paparazzi or a non-professional who insists that the whole world has to come to a stop so they can take the perfect photo is being just as rude to people who have legitimate reasons to need to go by.
So, when it comes to flash, I guess it depends on the situation. There are places that ban flash photography because of the damage the lighting can do to surroundings, such as museums. If a bypasser can tell that a professional or otherwise very serious photo shoot is in progress, I think s/he has a duty to interfere with that to the minimal extent possible. Thus, s/he should wait until the picture has been taken and then go by, or try to go around the other people. But a photographer should do his/her best to ensure that other people can go by with the least amount of detouring and waiting.