When they're laid off without cause, the often get severance, which is the equivalent of 2 weeks notice (actually often more; severance can often last for several months, depending on what you negotiate).
When they're fired for cause, it makes sense that they don't get notice, because typically to get fired for cause in the states, you have to do something that egregiously harms either the company or its customers. Think of the McDonalds employee that masturbated onto a customer's sandwich [1], or the Google employee who was caught perving on acquaintances by reading their GMail [2]. Do you really want them to continue working at their company, where they can do all sorts of mischief, particularly once they know they're about to get fired?
When they're fired for non-performance, it usually involves a whole paper trail of PIPs, documented non-performance, etc. This is your 2 weeks notice that you're about to get fired, and it gives you a chance to shape up and avert the firing.
[1] http://www.atlnightspots.com/woman-develops-herpes-from-mayo...
[2] http://gawker.com/5637234/gcreep-google-engineer-stalked-tee...