Actually, that's exactly what it means. At-will means that either party (the employer or employee) can end the employment for any reason, good or bad, or no reason at all. Anti-discrimination laws have amended this, so the reasons aren't allowed to be "because the person belonged to a protected class", and there are a few exceptions for things such as employers not being allowed to fire employees for refusing to take part in illegal activities, but it is otherwise largely intact and very well can be used to fire somebody discussing their salary.
NLRB v. Main Street Terrace Care Center 18 F.3d 531
There's also state-specific clauses, like California's Labor Code is #232.
As silly as it sounds, At-will employment lets you fire someone for no reason, but it doesn't let you fire someone for an illegal reason. You can't fire someone for taking a lunch break or vacation time which is protected by various labor laws, just like you can't fire someone for being black.
Again, to me, the question is with whom does the burden of proof rest?
I apologise; I am clearly not familiar with the current state of exceptions. I'm sure it's obvious that IANAL as well, but, outside of specific exceptions (of which salary disclosure is apparently one as of the last decade) at-will still means "any reason or no reason" (though, California isn't exactly the best example of "at-will"....).
Unless my knowledge in this case also happens to be woefully out of date, the burden of proof lies on the allegedly wrongfully terminated employee.
Well, the issue is that the company in an At-will state will never give any official reason for a termination. They will resist even giving a reason as long as they can and if forced there will always be something they can use.
