Firstly, I'm Canadian so I have my own issues with the French and French people in general - I'm not here to defend the French.
Secondly, I appreciate your position, but really you can't claim that Polanski is a "child rapist" in the legal sense. That is not what he was convicted of - he was convicted of "unlawful intercourse with a minor child". Now, before you get all ballistic, that's not my decision - that was the decision of the American judicial system at the time of his plea bargain and conviction in the 70s. If he had been convicted of "rape", he would have been serving a life sentence in a California prison, not a 90 day psychiatric assessment, as ordered by the judge in his trial.
Now, just as with Linsay Lohan and others in the California "bad celebrity" hand-slapping justice system, Polanski went to prison, served his time, and was released by the authorities in charge after he was evaluated as having no psychiatric issues and as no threat to society. You can argue that the "authorities in charge" didn't do a good enough job, but Polanski didn't issue his own release or break out of prison - he left when he was released.
I wasn't paying attention at the time, but I'll guarantee that the judge and prosecutors in this case got a lot of grief from the public regarding the leniency of this plea deal and sentence - rightly so. But in your system of justice, do you want judges after the fact, after getting public grief, to be able to go back and change their minds when they get pressured? Polanski fled after the judge issued a bench warrant to arrest him again after he was released from prison. That warrant, in most cases, would be illegal and perhaps was illegal here - Polanski wrongly made the decision not to challenge that arrest in court and fled the country instead. After 40 years, and many opportunities to arrest him previously, a prosecutor in 2009 decided to make a name for himself, likely for political reasons, and activated the extradition proceedings.