That’s a pretty good analogy, but it’s what you do with the detectors answer that’s the problem. If you just say: “The detect says no, we won’t accept your package”. Then there’s no problem.

Apple could do the same, simply letting the user know that this/these images cannot be uploaded to iCloud, and then do nothing else.

The problem is that the results of these scans are pretty useless. They don’t prove anything. While Apple knows that law enforcement and politicians will believe it’s 100% correct, because they don’t even understand that DNA can be wrong, and demand customer names from Apple.