Maybe they can take the step in actually not assuming that every living being in the entire universe celebrates Christmas, just a thought.
Well lets examine the Christmas episodees
"The Christmas Invasion"
Location: Earth, UK early 21st Century
"The Runaway Bride"
Location: Earth UK early 21st century
"Voyage of the Damned"
Location: In Orbit over Earth early 21st century. Visited UK
"The Next Doctor"
Location: Earth, UK mid ninteenth century
"The End of Time"
Location: Earth, UK early 21st century
"A Christmas Carol"
Location: Unknown
"The Doctor, the Widow and the Wardrobe"
Location: Earth, UK mid 20th century
"The Snowmen"
Location: Earth, UK late nonteenth century
"The Time of The Doctor"
Location: Trenzalore
So out of the 9 Christmas specials, only two ("A Christmas Carol", "The Time of The Doctor") have not been set on or around Earth or more specifically the UK.
Now whilst the UK might be more secular these days many of it's traditions come from the Christian beliefs. Walk down the majority of streets in the UK in the period around Christmas and you'll notice Christmas decorations.