I understand that most(?) Of the US is an "at will" jurisdiction, but where I'm from all employment contracts have a notice period in them. So when employment is terminated, by either party, there is a 2-4 week period where the employee works out their notice.
Often in the case of lay offs there would be plenty of heads up before the official notice period as well.
Only in the most egregious cases of misconduct could an employer get away without a notice period.