Even as the earthquake hit and all the doors of the prison opened and the prisoners inside fled, Paul and Silas stayed behind singing praises. Why?
Because they were where they were supposed to be. Yep, finding God while in prison isn't all that strange, but most of those people were sentenced to prison, be it for a period of years or for life or even death, and are there for a reason. Good that they found God there, but they still have a price to pay for their actions that led them to prison in the first place.
Or as a preacher I once knew liked to say, 'God forgives all sins. The judicial system isn't God.'