Which companies don't extend that courtesy? Any company I've worked at gives 1-3months notice for low performers. They call it a "Performance Improvement Plan" or somesuch, and basically pay you while you search for another job. For layoffs, there's a severance payment equivalent to several weeks. No salaried job at a going concern would skip severance, because severance is on condition of promising not to sue the company for anything.
Whenever I (or anyone else I've seen) have been laid off, and I've been laid off from several jobs, I would have about fifteen minutes to pack my things and say my goodbyes, and be ushered out the door right away.
People that resign may or may not be allowed to stay for the two weeks, depending on how much the company was relying on them (often the companies didn't really have much redundancy or documentation in place and needed to train someone new and/or do a few meetings with infodumps with these people)
