I don't really think H1B workers can be considered at will workers. The company usually retains the right to fire the worker at will, but H1B workers have limited rights to remain in the US should they leave their jobs without a new sponsor lined up.

US citizens and permanent residents can change jobs freely, drop out of the work force, go back to school, transition to a new career, and so forth, all without jeopardizing their right to live and work in the US. While H1B workers do have some rights to change jobs, their mobility is considerably more limited.

You don't have to be especially cynical to believe that the very one-sided nature of the "at-will" relationship here, along with the high stakes for the employee (if you can't get a new job with a new sponsor, you will have to leave the country), is huge part of the appeal of these visas.