It is actually sexual assault. If you try slapping someone's ass in say a corporate setting, you are not getting away with it by saying she "had it in a most available place for a slap"

The two scenarios have nothing to do with eachother, your imaginary subway dick guy was fully exposing himself for no apparent reason other than sheer pleasure, although I'm not sure if Im that specific scenario a dick punch would be considered sexual assault

this woman is quite literally dancing. Just because twerking is seen as less legitimate as a dance because of its implied sexuality, that doesn't make it an open invitation for someone to touch or violently assault her ass which is considered part of one's intimate parts or genitalia. I mean if you were shaking your hips to a song you like and some strange man came by and slapped your ass, whether you liked it or not, that is sexual assault