Earlier quoted context omitted.
Even if someone went browsing for it, yes that's illegal but there's no benefit in their therapist reporting them for just visiting terrible websites. But also there are definitely ways to get accidentally exposed. That's an absolutely awful thing to call the cops over.
I think you're demonstrating incredibly poor judgment here. CSAM is a crime with real victims. Even if your patient came across it innocently, someone is out there intentionally distributing it and that needs to be investigated.
A whistleblower goes to a therapist, stressed out over their pending decision to reveal official misconduct. They've been investigating ways to post something on the internet that can't be immediately taken down by the corrupt government officials they want to expose. They express their discomfort, in confidence, to their therapist, about using something they've discovered is also used for CSAM.
You think it's a good thing for the therapist to be required to report this? Should they report that the patient admitted to viewing CSAM with no context so the whistleblower gets investigated and arrested, or should they provide the context -- that the patient is about to expose the corruption of the government receiving the report?
For that matter, consider what it does when someone is actually a pedophile. They find out that if they try to seek therapy to address their perverse attraction to kids, the therapist isn't allowed to keep their confidence and they'll be arrested, so instead of seeking professional help, they keep abusing kids. Is that the result we wanted? There is a reason doctor-patient confidentiality was a thing.