That was an interesting question.  Thinking about it, I think set holidays have a benefit for the employer in that he can shut down operations on those days knowing there will be no workers.  If he gives the workers the option of taking Labor Day or the Muslim holiday, then probably about 700 of his 1200 workers wouldn't show up one day, which probably would mean his plant would not be able to operate, or would operate at diminished capacity, and about 500 workers would not show up on Labor Day, again making it difficult to operate at full capacity if at all.

So the set holiday means the employer can schedule down-time at the plant and at other times expect to run at peak capacity.