Wrong. Read up on WARN. There are conditions where an employer is required to give warning and others where no notice is required.
https://en.wikipedia.org/wiki/Worker_Adjustment_and_Retraini...
https://en.wikipedia.org/wiki/Worker_Adjustment_and_Retraini...
>An employer who violates the WARN provisions is liable to each employee for an amount equal to back pay and benefits for the period of the violation, up to 60 days.
...pretty much says companies can do what they want. For a lot of these folks, that's a severance.