Your doleta link talks about the WARN act, which requires advance notice of mass layoffs or plant closings, not severance pay. Did you link to the wrong thing?
It was originally legislated to protect workers from factory closure, but applies to many workers who get laid off without notice. Severance (though not in that name) is mandated in the Penalties section when due notice is not given: "An employer who violates the WARN provisions by ordering a plant closing or mass layoff without providing appropriate notice is liable to each aggrieved employee for an amount including back pay and benefits for the period of violation, up to 60 days."
I realize that the large severance pay typically given to IT employees is not intended to comply with this law (though it would would prove satisfactory in cases where WARN applies, should anyone investigate), but merely wanted to point out that it exists.