Violation of possession ban in DC was mandatory year in jail, Rowan served 365 days less than that. And that was WITH shooting the evil poolhopper.
The city did attempt to prosecute, and IIRC, the trial was highly publicized. It ended with a deadlocked jury and the judge declared a mistrial. I don't think that they treated him particularly special. His claim was that his son was a former federal law enforcement officer, and the weapon was exempt from DC law. Apparently, the DA's office disagreed. He was, FWIW, a total hypocrite however. He's dead now.