I can understand her fear as a fundamentalist: because once those evil Lieberal Evilutionists start teaching him that there are more holidays than Christian ones, who knows what could happen? He might realize that more than one religion claims it has the "One True God," thus helping to create world-wide problems. CULTURAL DISSONANCE, O NOES!
"....so I to tell him how some people haven't found Jesus.... Why do they do this?"
Wouldn't that be a good thing, for him to understand that not everyone is Christian? He can bring people to Jesus that way.
Why do they do this?
I thought that they weren't supposed to do this?
Probably for the reason I mention above, to teach him about the world and its various peoples, as is the job of a teacher/school. And only in a twisted, far-right Christian world view is it illegal to teach things that are not "Christian". Our Founding Fathers believed in a concept called SEPARATION OF CHURCH AND STATE, even if you believed that they were all really Judeo-Christian. (According to their own words, and not the words of others, many of the FF were Diests.)
Oh, and there are many holidays in December, world-wide, many depending upon different calenders than ours. Take a look at Wikipedia's Calender of Holidays; December is on the bottom of the page. Many holidays are in December, even some GASP! SECULAR ONES!
Oh, and birthdays in December too! Like mine! (Though mine's over for the year...) Those are personal holidays, aren't they?