So, what if you want to fire someone because they're just really annoying to be around, and their coworkers are complaining that they hate coming to work and dealing with that person? (This is commonly euphemized as a "lack of culture fit.")

There's no actual productivity loss in such a case; there's just a bunch of unhappy coworkers who have one fewer reason to stay when a head-hunter approaches them.

This is basically why companies fight for at-will employment: they want the ability to fire people for what are essentially petty, yet still important reasons, without having to ever say that sort of thing out loud. Because, you know, any company that actually admitted that it did that would be tarred in the press—not for doing it, but for failing to be discreet about it.