Crossing that line destroys the plausible deniability surveillance enjoys. You can't make the 'necessary for terrorism' argument when the surveillance directly subverts the domestic government. And of course thats what we were all worried about, and it's the most arresting sort of example for everyone. And how did that 'unwritten rule' become a thing in the first place?
A lot of the wide appeal and scary implications that could be written about gets lost when gloating about how lemmings 'flip-flopped' after they walked into that machine that rips off their heads.