https://en.wikipedia.org/wiki/At-will_employment
As of October 2000,[27] 42 U.S. states and the District of Columbia recognize public policy as an exception to the at-will rule.[28]
https://en.wikipedia.org/wiki/At-will_employment
Public policy exception is more to do with preventing employees from being fired if they refuse to do something their employer asks which is illegal, or defined as immoral or suchlike. For example; If an employee reports their employer for tax fraud, or being involved in organised crime, would be examples of where an employee would be protected by public policy doctrine and couldn't be fired. Or even silly things - like somebody being on Jury service for an extended period - you couldn't fire someone for that.
In the general sense, you can still be fired pretty much instantly for pretty much any reason, and on the spot. In contrast to the EU where it's very hard to fire somebody.