The proliferation of CSAM is extremely harmful to the victims in the photos and more people seeing them might encourage more CSAM production in general.
To be clear, I'm not disagreeing with you in regards to the harm caused in the specific case of CSAM, but I can't help but see this as a slippery slope into having the ability in the future to label and act on other kinds of information detected as objectionable (and by whom one must also ask), which is itself incredibly harmful to privacy and to a functioning free society.