I cant help but notice that, most of the people with the strongest opinions seem to actually reside in areas, where events such as these are likely to take place. Of course I am just going by the profiles they list but I think when you live somewhere where you see this kind of sht every day, it affects your opinion greatly. Personally, I can't imagine ever being in this situation because if I stopped at a McD's and the clientele appeared to be of a certain demographic, I would promptly GTFO of there and go somewhere else.
I don't mean to imply anything racial but more so the probable education level and just general "ghetto" mentality these patrons would be likely to display. If they all had nice church clothes on or business attire and spoke with manners, I would feel much better about where I was at.
That being said, if I was there, I would have very authoritatively commanded them to stop. If no positive result came of that, at 6'3" 250 lbs (not bragging) I would probably have grabbed both assailants by the throat and clunked their fking heads together and threw them out the door. If they then presented weapons or persisted in any way (unlikely), I would have taken it to the next level.
I know this sounds like some macho BS but I have done it before to grown men and not only does it work but there is usually a moment of disbelief by the perps that someone actually just did that to them. Most people who attack like this only do it because they know they can beat the weaker opponent. A good knowledge of pressure points of the human body can be a very effective ally when dealing with folks such as these.