>The only thing stopping your phone from keylogging your password to a server in the NSA somewhere if it recognizes a specific trigger pattern is Apple's willingness and ability to resist pressure from the US, etc.
Think of what would happen if you tried to make your average Silicon Valley dev team design, implement, and test a surveillance system they didn't want to build and that was immoral. They'd resit in an infinite number of ways that would delay the project virtually for ever. Short of summary executions, I bet you could not get a nice, efficient, effective system.
On the other hand, once the dev team has enthusiastically built the system that scans for any image, it's entirely easy to say "Now, make it look for these images." They have no avenue for resistance other and a up front no. And a government that wants to do totalitarian things knows many ways to force a yes.