Why are people answering this question without even reading up on the case?
He pled guilty to unlawful sex, that is already settled. However, there were irregularities in the trial that seem likely to cause it to be overturned on appeal.
Time passes, and it does make a difference. Jail sentences are intended to protect the community and to rehabilitate the offender. In this case, decades later, we can see with 20-20 hindsight that the community needed no further protection and the offender needed no further rehabilitation. No point would be served by a jail sentence.
We specifically also know:
"Polanski has asked a U.S. appeals court in California to overturn a judges' refusal to throw out his case. He claims misconduct by the now-deceased judge who had arranged a plea bargain and then reneged on it.

His victim, Samantha Geimer, who long ago identified herself publicly, has joined in Polanski's bid for dismissal, saying she wants the case to be over. She sued Polanski and reached an undisclosed settlement.

Earlier this year, Superior Court Judge Peter Espinoza in Los Angeles dismissed Polanski's bid to throw out the case because the director failed to appear in court but said there was "substantial misconduct" in the handling of the original case." Associated Press article

So we even know that the victim of the unlawful seduction has been compensated by a civil settlement and has long forgiven him.
France and Poland (he is a dual citizen of both countries) have protested the arrest.

What we don't know is why, in 2005, the US Marshals dug up this case, decades old even then, and started pursuing it. One can only suspect that the Bush administration was trying to score points with the social conservatives. Now our current administration finds itself with a completely unecessary diplomatic row when it already has a full plate of vastly more serious matters.