I'm still not sure if the general public has understood this, or its me, my initial thought was the bloke shouting 'youaintnomuslimbruv' was saying it simply because the attacker wasn't a Muslim. Simple as that, he was just pointing out, you aren't even Muslim so why are you doing this.
But was this coming from another Muslim who was condemning the guy? If so then I get the meaning. In other words I'm Muslim too and I'm ashamed you, you aren't a real Muslim.
There is this whole double negative thing going on, and I honesty am not sure what was meant by this.