I'd also like to see some more details about:

In the U.S. “at-will” employment is permitted by law in most states.

In which states is at-will employment not "permitted"?

My understanding of at-will employment is that both employer and employee - in the absence of an employment contract - have the right to terminate the employment agreement at any time, for any reason, or none at all (with the exceptions being employers cannot terminate someone for illegal reasons, many of which are spelled out in the Civil Rights Act of 1964).

If the employer decides to terminate for any legal reason which is not "for cause" (where the employee committed a terminable infraction), the employee is entitled to receive unemployment insurance benefits, a good portion of which comes out of the employer's pocket.