Mandatory reporting makes sense for situations you are connected to. And even then there's presumably good reasons not everyone is a mandatory reporter. This goes way beyond that, mandatory reporting once removed for someone that doesn't know a single person involved.

Hell, reporting someone for that doesn't even guarantee the images get looked into! If they didn't save history they're probably not feeling like going back to the site to demonstrate. Similar if it was sent against their will and they deleted it right away.

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.