How are they measuring this?

What an incredibly simple idea. Just scale it up.

FTC says “[..] it is an unfair method of competition—and therefore a violation of section 5” in their rule.

Indeed, the act gives them the ability to regulate unfair practices, but it’s not clear that this is unfair. Is it unfair if I get paid a good salary in return for not working for competitors? If they pay me for the years I can’t work after resigning, I would very much say no. If they don’t pay me for those years, I would say it could be seen as being unfair. But the rule is a blanket ban on all noncompetes essentially, even for senior executives! (Unless I’m reading it wrong.)

I’m for banning noncompetes, but I don’t think you can reasonably argue that the FTC has this ability.

I do think they could ban the subset that are clearly unfair without any issues FWIW, and I do hope they switch to this direction.

Even then, it wouldn’t hold in intrastate cases since the Commerce clause doesn’t provide for that.