Pedophiles who are concerned they might offend against children with low probability know that if they tell a therapist about their attraction, they might be reported, if the therapist decides they are an imminent danger. Most pedophiles don't know what criteria their therapist would use, they don't want to risk it, so they do not seek help.

Robin Hanson or Eliezer Yudkowsky made a post on this, with terrorism substituted for pedophilia. The benefit of having a therapist able to apply influence to the individual would come from the commitment to privacy. As with priests confessionals, etc.

If the choice is between a potential perpetrator talking to a therapist and having a chance of being influenced but not reported and a potential perpetrator speaking to no one then the consequences are in favour of mandated silence... unless most perpetrators are somehow stupid enough to effectively confess to their impulses to the police for the hell of it.