No, unfortunately, depending on the state, especially if it's an at will state, it isn't illegal. Those states are not required to give bereavement leave, which I think is unfair, someone just lost a person they care about, have some compassion. However, the boss is justified on the firing in this case if they don't allow leave for it, it's not right, but it is legal.

Unless OP is an idiot, it seems that he was told that he has the ability to alter schedules. Which, if that's the case, it would make OP's boss a asshole. And, again, something put OP under the impression that he had the authority to make that decision, so barring OP just assuming he does, it seems like this could very well be a way for the boss to get rid of people they don't like, and use OP as the scapegoat. That said, is there anyway that you could talk the boss down? Like under the statement of it having been your mistake, etc. Obviously I'm not telling you to endanger your career over it, but it's worth some consideration.