I’m not sure why you think there’s no role for the bishop in such cases. If a priest’s crimes are reported to police, the police will investigate. Then the state’s attorney will determine whether the halting and sometimes contradictory testimony of a shy child or two (whose continued involvement may be opposed by their parents) is enough to convict a respected and popular community leader, the parish priest. In many cases, the answer is no. According to you, it’s up to the law, so I guess no further action by the bisohp is needed.
The reality is that bishops often know what is going on before there is sufficient evidence to convict. They have a moral obligation to take what action they can to protect their flock from abuser priests of whatever stripe. Unfortunately, the Church made such actions more difficult than they should have been.
We definitely have to credit Cardinal Ratzinger for seeing the need for a change in rules, and for achieving such a change. However, I think there are grounds for criticizing how long he took to do that, and I definitely think there are grounds to criticize the inefficiency and inertia of the curia that John Paul presided over, that made Ratzinger’s job so hard.