It's a cross-section of devout Marxists and young, dumb and full of *** kids who think anarchy means being able to do drugs and have sex whenever you want without having a job. Much the same as the Vietnam war protests, the No-Nukes movement, the anti-globalization crowd, the Greens, etc, etc...
Mumia is just a rallying point for the true believers and their useful idiots. The murdered police officer is their conquest.