I'm not too familiar with his trial and the situation.
It's a pretty bad situation. Polanski originally agreed a deal where they would drop most of the charges in exchange for a single guilty plea of unlawful sex with a minor, and it seems a deal was done for him to him to get probation on condition that he left the USA. The district attorney then met the judge privately, and convinced him to actually put Polanski back to prison, which was a crime under some sort of law designed to stop judges being influenced.
For me, no-one comes out of this looking good, but the general argument against deporting him is that with all the media attention, it will be nearly impossible for him to get a fair trial.
One odd thing about the story - Polanski was photographing her topless. Where the hell were her parents? :/