If I ask you to store my images, and you therefore have access to the images, you can scan them for stuff using your computers. The scope is limited to the images I ask you to store, and your computers are doing what you ask them to.

If you reprogram my computer to scan my images stored on my computer… different thing entirely. I don't have a problem with checking them for child abuse (in fact, I'd give up quite a bit of freedom to stop that), but nothing about this tech makes it specific to child abuse. I don't want my computer ratting me out for stuff that I have the right (or, possibly, the obligation) to be doing, just because the powerful don't want me doing it. At the moment, it doesn't.

This tech makes Apple-controlled computers untrustworthy. It will probably lead to the deaths of political dissidents; these things always do. Is that worth it?