Yes. Depending on the state you live in, they may be required to pay you for that shift.

I think the ethics of the situation revolve around the specifics of the situation. If there's an explosive situation where you cannot or do not want that employee in your building due to his or her actions then I say yes, it's ethical. If you've been building a case against continuing to employ someone, I think it's best practice to bring them in at work, be frank with them, have them clear out their desk, and escort them out rather than have them show up and fire them before they even enter the building. But no firings I've ever witnessed have been pleasant situations, so maybe I'm biased.