In a situation like that, we're not talking about cold, rational threat assessment. You're dealing with instinctive assessments of, as another commenter said, power balance, from a part of your brain that's a) very concerned with physical survival and also b) attuned to the support of other people around. And if the guy got away with putting his hand on her leg in public, so reasons the lower brain, who knows what else he can get away with?

Disclaimer: I'm a man so I've never dealt with that exact situation, but I've observed the above thought processes in awkward situations, and I would assume it's amplified in the described situation.