Why not? It's a normal working day in the US. (Well, in principle -- but in years when 25 Dec is on a Thursday, for example, then 26 Dec may be also be considered an official выходной день, in order to create a "four-day weekend".) In other words, the "Boxing Day" holiday is not a universal Christian custom.
And don't forget about the difference between the Gregorian and Julian calendars -- for Russian Orthodox Christians, who still use the old Julian calendar for reckoning holy days, both 25 and 26 December are simply two ordinary days in the middle of the long pre-Christmas "Advent" period. (A 40-day season that they call the Рождественский пост, "Nativity Fast", and in some ways it's more similar to "Lent" than to Western "Advent".)