Because not every photo is uploaded to iCloud. If you use iCloud, this is already happening today, probably. If you don’t use iCloud, yes, the idea is local s

Because not every photo is uploaded to iCloud. If you use iCloud, this is already happening today, probably. If you don’t use iCloud, yes, the idea is local scan, match hash, then upload for manual review. The involuntary upload will have been covered by the EULA. I’m not endorsing the idea. But this seems to be the only way it could work.