Why wouldn't it be legal? Virtually every US state has zero notice requirements -- tell your employee that they are being fired, laid off or otherwise no longer going to work here, pay them what you owe them immediately, and you're done.
"Two weeks notice" is a social nicety, good for maintaining good relationships and status. If you don't care about those things, you can quit or fire on the spot.
"Two weeks notice" is a social nicety, good for maintaining good relationships and status. If you don't care about those things, you can quit or fire on the spot.