I don't think that's really how job offers and at-will employment work. An employer can terminate an employee at any time (even before they've started) as long

I don't think that's really how job offers and at-will employment work. An employer can terminate an employee at any time (even before they've started) as long as the cause for termination is not illegal. "Employment contracts" are contracts, and a stipulation of that contract is the at-will nature of the employment.