Why do people prefer the scanning on the cloud storage?
That means it will never be encrypted and stored unencrypted on HDD's in someone else's computer.
'scan -> encrypt -> upload' is in my opinion better than 'upload -> scan'
Which would justify Apple using on-device scanning more if iCloud is end-to-end encrypted, except that it isn't. Apple has the technical capability to decrypt photos stored on iCloud, so why risk the slippery slope and governments applying pressure to expand local scanning to more than just what is going to be uploaded.
'scan -> encrypt -> upload' is in my opinion better than 'upload -> scan'