Two arguments in support of Roman Polanski, as I understand them are:
Firstly, it is primarily a cultural difference, since sex with a 13 year old is more accepted in Europe. According to this argument, Mr. Polanski's actions are, at worst, the result a cultural misunderstanding that should mitigate against any severe punishment. Most carry the line of argument further and claim that it shows that the charges reflect the immaturity and prudishness of the United States.
According to Wikipedia "The ages of consent for sexual activity vary by jurisdiction across Europe. Spain sets its age of consent at 13, and the rest of the countries have an age of consent between 14 and 17, except Turkey and Malta, which have the highest age limit, set at 18.".
See: http://en.wikipedia.org/wiki/Ages_of_consent_in_Europe
Secondly, The victim in the assault case herself has asked that the case against Mr. Polanski be dropped. If she does not want him punished, why should we?
I have concerns about each of these rationales.
As to the first, even in Spain, 13 is the age of "consent." According to the charges, the victim in this case did not consent. She was, by the accounts I have read, drugged and raped.
As to the second rationale, I believe that the victim wants the charges dropped, not because she feels that it is inappropriate to punish Mr. Polanski further. Rather, she feels that the publicity that would accompany a trial would be a further burden on her. I recall a quote by her something to the effect that what Polanski did was hurtful and gross, but it was the news media that ruined her life. This does not argue that Polanski is being punished too severely by a prudish, barbaric American legal system. Rather, illustrates once again the lack of moral compass in our American media.
