I think this would be more like if the TSA installed scanners in everybody's houses that scan everything as you put it in your suitcase. Yeah, it only scans things on the way to the airport, but the scanners are in my house. I don't want them there. My house is my domain.
It's the same way with my phone. I don't want the government/Apple evil detector running on my phone. I know that a false positive is unlikely and I know that Apple and the government have made very clear they will only look for Real Evil. I still don't want the scanners on my phone.
This indicates you may be misunderstanding why people generally feel better about things happening on device.
Apple's own tagline "What happens on your iPhone, stays on your iPhone." includes a very strong hint. That hint is:
> Stays on your iPhone
Nothing about the features we're discussing here stay on your iPhone. Doing something locally is no longer meaningful if the express intent of doing that thing is to phone home when certain conditions are met.
Yeah it IS different, most people feel much better about things that happen on device.
Since this only happens for photos on their way to iCloud, in your analogy it would be like searching the bags when you put them in the cab to go to the airport.
Furthermore, it would be like TSA had a machine that could search your bags for guns, without any human even seeing an x-ray of them. Plus the machine would need 30 gun alerts before it takes any action at all.
I don't know about you but I would not have any issues whatsoever with that.