Every AV (antivirus) software system scans customer-owned files on customer-owned devices. So the question is the same: if it’s easy for law enforcement to deputize such a system, where is the flood of cases built off of evidence gathered this way?
This is not about child porn nor about what AV software can or cannot do.
It's about normalising mass surveillance and implementing populace control. They don't want another Snowden to scare the public with reports about "backdoors" and mass privacy violations.
They want the coming generation to perceive it as normal. Because all phones do it, hasn't it always been this way, and think of the children.
Oh, these dissidents in $distant_country that will be muted and killed using Apple's shiny surveillance tech? Well, evil governments do what they do. But we are not like that. Over here it is only about the child predators. Trust us.
Apple has been an opponent of these developments for decades. Now they are spearheading it.
Mine doesn't. If Ubuntu or ClamAV is scanning all my photos and reporting the results to Canonical against my will then I will soon be having words with Mr. Linus.
Because if some local police agency like the FBI were altering code on an open source project as large as ubuntu then the world would know about it pretty quickly. The linux community would burn every bridge with Canonical and ubuntu would disappear overnight. The idea of the FBI adding malware to ubuntu unnoticed, malware that openly scans files and reports to remote servers, is the stuff of comic books.