How are we meant to know this is "rude behaviour?" unless we are privy to allot of information about their personal lives they could have a previous relationship, could be married, or be hooking up.
If I see a guy put his hand around a women I am not going to try to intervene because I don't know the background there. It is just unrealistic to suggest people should do otherwise.
Now if she had previously mentioned this behaviour or the individual in a negative way I might say something, but based on the situation (and most situations) that wasn't the case.