You're correct, in a free market the union could offer better incentives. They could also just say, "Whoops, we messed up the leave schedule. You now have to

You're correct, in a free market the union could offer better incentives. They could also just say, "Whoops, we messed up the leave schedule. You now have to work the holidays or you're fired." I suspect a combination of the two, some carrot and some stick, would happen without the union contract.