I think it is fine that you opt out. But if society as a whole doesn't opt out of a surveillance regime, it still affects you as a member of society. And eventually your ability to opt out will probably be removed: after all, once you've acquiesced to the idea that private, unshared, possibly encrypted photos still need to be scanned for criminal activity -- then why should there be an exception for criminals who back up to a local computer.

Because in the USA if we keep our secrets in our house we are protected from unreasonable search an seizure. If we share our secrets with a third party the third party is free to share them. This seems like such a clear line of distinction that a layperson could understand.

There is no U.S. law that requires Apple to scan your uploaded content for CSAM and share reports with law enforcement. If such a law existed, it would almost certainly be a major 4th amendment concern. Instead Apple chooses to do server-side scanning "voluntarily" (which means, they do it under considerable pressure from law enforcement agencies and politicians) after making you click Agree on a ToS that gives them permission to do so.

Similarly, I am not aware of any constitutional rule that prevents a private company like Apple from installing client-side software into your phone OS that scans the local photos on your private device even if you never upload them, as long they don't do the scanning at the government's direct behest and you give them permission by agreeing to their ToS.

The important constitutional principle here is "is this private company acting as an agent of the government." The important civil principle is: "do we, as citizens and customers, want our government and phone manufacturers to scan private files and submit automatic police reports." Once you've decided those questions, the physical/logical location of the files is a detail that only we nerds care about.

Never said there was. I said there is a constitutional idea that items stored at home have more expectation of privacy than things store with other people. That you have some axe to grind with this or that company shouldn’t cloud the issues.

You're trying to argue in good faith with someone that fully understands the strategy of using private/public partnerships to bypass constitutional protections, and is playing oblivious. Don't feed the glowies.