You might want to re-write that passage, because I read it the same way Ambridge did; that you were saying they were fired for simply saying they were going to protest. Nobody was actually fired until they didn’t show up for work.
And yes, while many businesses allow unpaid time off for arbitrary personal reasons, they usually have you put in a request in advance. If SO many people do it on the same day that the business can’t operate, I think it’s reasonable for the owner to threaten termination if they don’t show up on a day the leave was denied. The reason so many employees decided to not work that day isn’t really relevant.