It’s a difference in policy vs technical capability. Currently the policy is only scan when iCloud Photos is enabled, but the capability to scan at any/all times is just a policy change away.
No, it's a difference between scanning the files that users store in their respective clouds on-server or on-device.
Scanning on-device (where a single false positive cannot be subpoenaed and misused to incriminate their customers) is simply more private.
>Innocent man, 23, sues Arizona police for $1.5million after being arrested for murder and jailed for six days when Google's GPS tracker wrongly placed him at the scene of the 2018 crime