No one trusts the US justice system any more, and the French have never trusted it. More people have been screwed in the US justice system than any where else in the western world. I know my own country refuses to extradite people to the US now days without being presented with all the evidence and even then it is far from sure. Why? because of several cases where a person was extradited on certain conditions and then the prosecutor went back on those conditions when the person landed on US soil.
And in the Roman Polanski case.. he served his time.