I think the comparison to a leech is accurate, as the leech does not know or understand it is a bloodsucking parasite, but continues to steal blood and nutrients, those ignorant of the work unions perform will benefit from them regardless of their contribution.
Rarely do you find someone who both genuinely understands what a union is for and declines to join when offered the opportunity.
Diehard is 100% a Christmas movie, however, Die Hard 2 which also takes place at Christmas time, is NOT a Christmas movie, as the fact that it's Christmas is coincidental and not at all integral to the plot.