I am not saying this to in any way excuse Alice Cooper, but at that time (remember Polanski) far too many young girls, and I mean girls, were encourage to think that any sort of attention from famous men was an achievement. And suspect many of them were raped as they would not have had the concept of what these men wanted from them. Look at the Rolling Stone that married someone he had been "dating" since she was 13. That would be called grooming nowadays
So again, not an excuse, but doubt many men would have thought there was an issue. And am only saying this as a reminder of just how much campaigning and raising awareness 70s feminists had to do at that time to even get the issue discussed.
I doubt if every male rock or film star had to be truthful would be able to say they hadn't had sex with an underage girl.
Not forgetting fashion photographers, and ...