I suppose one important distinction here, if this makes any difference, is that drugs and weapons (to use your examples) are physical items, and these could arguably cause harm to people downstream, including to the postal system itself and to those working in it. In contrast, photos, text, written letters, and the transmissions of such are merely information and arguably not dangerous in this context (unless one is pro-censorship).
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.
I suppose I should clarify my point that I was referring to those dangerous items in the mail in the sense of their capacity to directly cause physical harm to those handling or receiving them, rather than in the more general sense of the societal effects of their proliferation, which is something else altogether (to your point).
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.
