The main reason he's in prison today is his own stupidity. What people don't often mention is that he represented himself in court and instead of putting on a real defense he grandstanded on the MOVE house bombing by the Philadelphia police. And he could very well have committed the crime. The stuff about him being under surveillance since he was a teenager is bullshit; yes, he was in fact monitored because he was a member of the Black Panther Party, but so were many other people. Mumia has distinguished himself while in prison by writing several shitty books and casting himself as a martyr. If you really want to free a political prisoner, free Leonard Peltier. And commute Mumia's sentence to life in prison.
*on edit: ok, Mumia didn't represent himself throughout the trial although he did it at the beginning. I still maintain that Mumia did things at his trial that sabotaged it.