What Apple is doing is the equivalent of the police one day deciding to search everyone’s physical home photo albums just in case there’s a picture of an illegal activity.
They only do the scanning if you use iCloud Photos backup. If you use, say, local storage instead of iCloud, you don't get scanned (which would be a more apt comparison to a physical photo book). Also, they aren't scanning for new illegal activity, but images that you somehow obtained of past illegal activity that the government already knows about.
This to me is the most unbelievable aspect of the whole thing.
Apple is basically saying:
"So hey, if you have anything you don't want us to know about, here's how to get around our scans (wink, wink)."
What would be the point of implementing a system and then telling people how to avoid it? I think at some point in the near future they will flip a switch and start scanning private photos.
They only do the scanning if you use iCloud Photos backup. If you use, say, local storage instead of iCloud, you don't get scanned (which would be a more apt comparison to a physical photo book). Also, they aren't scanning for new illegal activity, but images that you somehow obtained of past illegal activity that the government already knows about.