"Criminal law involves the prosecution of wrongful acts by the state which are considered to be so serious that they are a breach of the sovereign's peace (and cannot be deterred or remedied by mere lawsuits between private parties)." - from Wikipedia
So, rape of a minor is definitely a matter for the state to concern itself with, as it has happened in the Polanski case. So, even if the victim forgives the perpetrator, the state (the government) can & should continue to seek justice on behalf of general society or attempt to apply the punishments that fit the crime. So that's what's happening now in the Polanski case - the legal conflict is now the state vs. Polanski, not victim vs. Polanski (or, it does not matter if the victim forgives Polanski or not, the nature of the crime makes it one that was committed against the entire state, not just one individual).
Anyway, that's how I understand it at the moment,…
1. The crime happened. All parties agreed that it happened. A child was raped, Polanski did it.
2. Polanski fled the country while awaiting sentencing. So, the legal, correct, & useful thing to do is to bring him back to face a judge, complete the sentencing phase.
3. Why? To reassure the nation that even talented, possibly wealthy, & well connected individuals cannot get away with raping a child.
And if there are other factors to consider, such as the original judge in the case changing his mind about a plea arrangement, those can be discussed during Polanski's new sentencing hearing.
A horrible thing happened, a crime was committed (a rape) within a country that has a legal code that punishes for such activities. To exclude Polanski from punishment would not be fair for everyone else - including other criminals who were punished for raping children, and rest of society that follows the law.