Nick,
Aggravate them - put up the tree for each of your holidays. When they ask why you do it, tell them it was their idea - it's a
"Holiday Tree". And if they get mad and say you can't do that, tell them they are discriminating against your celebrating the holiday - which ever one it is.
When someone tells me "Happy Holidays", I usually ask, "Which Holiday? MLK Birthday? Memorial Day? Which one??" Or "Seasons Greetings" - "Which season?? Summer? Winter? Spring? Autumn? "