Objectively she shouldn't let him do it. What he's doing is wrong, dirty and invasive, and she should speak up and say something to stop it. He's treating her as nothing more than an object. And by not saying anything, she's confirming that to him.
But a lot of women get embarrassed in these situations and freeze up. They don't want anyone to know. They're confused. They don't know who the man is, and they're afraid to accuse the wrong person, so they just stand there and take it. And possibly, even though it's weird, they're enjoying, which makes them feel like part of a dirty little conspiracy.