However, the public doesn't (and shouldn't) have the ability to inspect these images to verify that non-CSAM politically threatening images are not included. (ex. pictures of politicians doing cocaine with adult prostitutes, or images of war crimes committed by US troops)

I think that distinction is important. Because even if the NCMEC database is only child porn right now (which we cannot know), the public will also not know if the scope of the database later changes because now Apple will call the police on whoever possesses an image that matches this database. Adding this capability increases the incentive for misuse of the database.