It isn't for religious institutions to decide whether or not an accusation is valid. That's the purview of the criminal justice system. The role of the diocese is to determine whether or not a charge is credible. Credibility, in this case, means that a disinterested person would look at the facts as reported and determine that there is reasonable cause for investigation.

The problem is, bishops aren't disinterested. They're products of a deeply rooted clerical culture that considers any priest a valuable commodity that must be salvaged if at all possible. That is why bishops should be removed from the equation and should defer to review boards comprised of competent lay people.