Once the scanning feature is there, it just needs some small changes to target anyone the gov wants, not just criminals.
The same is true of the Photo app. It has been on devices since 2007 and just some small changes and it can send information about _interesting_ photos much more effectively to the government than the CSAM detection system.
And yet, it has happened.
Why would a government use an ineffective system, when Photos are much more effective?