People stop it from happening (to some degree and at great cost) because this would hurt far more than it would help. Imagine if the police could do this now. They would use it to drag everyone who had been in the vicinity of the protests out of their homes and lock them up (at best). "Secure" and "court orders" would never hold up in practice, but "permanent" certainly would. Plus, all criminals would need to do to avoid it is not bring cell phones to crimes, or bring someone else's. Unless you want to mandate carrying government tracking beacons at all times? I'm no libertarian, but this is one of the worst proposals for fixing police brutality I've ever seen.