Uh, you don't even have the story right. He offered her the pill, and told her what it was.
Yes, it's technically "rape" because we have age of consent laws. But in a bizarre twist, we have laws that state if both of them are 13, neither one is guilty of rape, or if one of them is 19 and the other 16, they are close enough in age to where it isn't rape.
Most of these cases are resolved without jail time, usually, they are just put on the offender registry (which of course makes these registries kind of useless, because they don't tell you who the actual rapists are.)
He served the sentence that the judge agreed to. The judge tried to do backsies.
Another part of the story you are probably unfamiliar with because of your ignorance and learning disability. Before Polanski reported to prison, he asked this judge for permission to go to Europe and finish filming a movie. The judge, who apparently changed his mind about what a horrible threat Polanski was, was happy to let him fly to Europe, where he put the film in the can and then flew back to CA to serve that 90 day sentence.
It was only after the media started roasting this judge in the press did he rethink is position.