I think one of the major factors that changes how people perceive this is that it's happen on their own device. If you upload a thing to a server and the server

I think one of the major factors that changes how people perceive this is that it's happen on their own device. If you upload a thing to a server and the server does something... I mean sure. You gave a thing to somebody else, and they did something with it. That's a very understandable and largely accepted situation. This is different. This is your own device doing that thing, out of your control. Alright sure, it's doing the same thing as the other server did and under the same circumstances* so maybe functionally nothing has changed. But the philosophical difference is quite huge between somebody else's server watching over what you upload and your own device doing it. I'm struggling to come up with a good analogy. The closest I can really think of is the difference between a reasonably trusted work friend and your own family member reporting you to the authorities for suspicious behavior in your workplace and home respectively. The end result is the same, but I suspect few people would feel the same about those situations. * There is no inherent limitation for your own device to only be able to check photos you upload to iCloud. There is however such a limitation for the iCloud servers. A very reasonably and potentially functional difference is the ability for this surveillance to be easily expanded beyond iCloud uploads in the future.