Here is the simple Wiki summary of the facts to avoid bias on either side of the issue:
In 1977, Polanski was arrested in Los Angeles and pleaded guilty to "unlawful sexual intercourse with a minor", a 13-year-old girl (he was 44 years old at the time). Released after a 42-day psychiatric evaluation, Polanski fled to France, has had a U.S. arrest warrant outstanding since 1978, and an international arrest warrant since 2005. Polanski for many years avoided visits to countries that were likely to extradite him, such as the United Kingdom, and traveled mostly between France, where he resides, and Poland. As a French citizen, he was protected in France by the country's limited extradition with the U.S. On September 26, 2009, he was arrested, at the request of U.S. authorities, by Swiss police, on arrival at Zürich Airport while trying to enter Switzerland to pick up a lifetime achievement "Golden Icon Award" from the Zurich Film Festival.
The sticking point seems to be that there was a plea bargain for a 42 day sentence the original judge seemed to be going to renege ( go back ) on. So Polanski fled the country to avoid a harsher sentence he thought was coming.
The question: should Roman Polanski return to face the court system?