I think the question is framed such as it already assumes the employer perspective: you should only get off on federal holidays, or at least religious ones.

But if you want some justifications: christmas is the biggest holiday in the (once?) christian-dominated west, and in the US in particular it is quite common for family to be separated by hundreds or even thousands of miles of travel. It is also common practice for people to celebrate christmas eve with one side of the family and christmas day with the other, if they live closely enough to each.

All that aside, it is less about not getting christmas eve off than it is about there being absolutely no reason for us to be there at all. The company recognizes that there is some importance to the day for its employees by cutting the day in half in the first place, yet even though all evidence shows that there is no reason whatsoever to be there and it's costing the company more to have people there than it will ever make from it, they still insist that people show up. It's not even rational slave-driving.