Looks like they have the sense to list Christmas on some of those games.
The difference in the games I posted and the snowman game is that the Author games are designed to educate and the snowman game is designed just to be silly. If you play the snowman game you will see just how silly it is.
It is possible that since it is a silly game, the designers deliberately kept all religious holidays out of it so as not to offend someone.
I am not defending PBS nor do I believe that they are any friend to Christians, but I do think that Christmas being omitted from this snowman game has nothing to do with any type of agenda.