Garrison Keillor seems to have generated quite an outrage in his piece, Nonbelievers, please leave Christmas alone:

If you don't believe Jesus was God, OK, go write your own damn "Silent Night" and leave ours alone. This is spiritual piracy and cultural elitism, and we Christians have stood for it long enough. And all those lousy holiday songs by Jewish guys that trash up the malls every year, Rudolph and the chestnuts and the rest of that dreck. Did one of our guys write "Grab your loafers, come along if you wanna, and we'll blow that shofar for Rosh Hashanah? No, we didn't.

Christmas is a Christian holiday - if you're not in the club, then buzz off. Celebrate Yule instead or dance around in druid robes for the solstice. Go light a big log, go wassailing and falalaing until you fall down, eat figgy pudding until you puke, but don't mess with the Messiah.

At first I thought Keillor was only mocking, by exaggeration, the "War On Christmas" hysteria by imitating it. 

His piece brought out a lot of comments earnestly explaining things to him that he surely already knows, about the non-Christian origin and ecumenically broad nature of today's Christmas celebrations.  Much of it seems to assume that he is the kind of "modern traditionalist" who loves "Frosty the Snowman" and only requires that we say Christmas rather than Holiday and that merriment and niceness and good will toward men is what Christmas is all about.  For example:

In all the years I've listened to Prairie Home Companion, and enjoyed Garrison Keillor's folky tales of an almost mythical midwest that I, as a Californian, could only vaguely imagine, I never suspected this streak of mean-spiritedness.

and

Wow, what a diatribe. Doesn't he understand that the "non-religious" bonhomie that is created by the songs, the decorations, the shared sense of frenzy to make OTHERS happy at Christmas probably does more to promote "good will among men" then any church service has ever done.

But if you listen to his show, you notice that he sticks with either the more-traditional Christmas songs, in the original languages, or modern songs that are doctrinally compatible with his beliefs.  And it takes a particularly tin ear to think that Keillor's prose unquestioningly endorses the supposed homespun values of the parents and authority figures of his youth.

So "leave my religion alone" is, I think, at core intentional.  

However, I'm not sure it should be taken as "don't sing those songs."  It's more, "don't tell me what Christmas is to me."  His piece actually tells people to go on and have Yule ceremonies or whatever they like; he doesn't seem upset that people with different views would celebrate at this time of year and "ruin" Christmas for anyone else.

He's invested in an internally-defined authenticity.  And he's not alone.  For many, religion is by its nature personal and irrational, not a social thing.  In MBTI terms, think introverted Feeling.

To those people, I as an atheist have no more of a right to tell someone that Christmas is about being loving to others in a time of need than they have to tell me that this season is only about the gift of the Christ child by a loving God and His eventual living sacrifice for my sins.  We have our own beliefs and hands off.

The "real spirit of Christmas" arguments can be used to obliterate a lot of what it means to people who really are very religious, just as the commercialization of Christmas can be used to obliterate a lot of what it means to people who aren't religious but are into the spirit of love and sharing, and the "Jesus is the reason for the season" bit can be used to obliterate what it means to people who aren't religious.  It goes full circle.

It isn't just Christmas.  Non-Christians telling Christians that they should read Matthew 6:5-6 on praying in public are just as annoying as Christians telling non-Christians to read Leviticus on homosexuality.  Claiming there's no such thing as a sin or a sinner, just mistakes and people, is literally doctrinally incorrect for many religions.

In short, there is some of the same instinct to totalitarianism in the smiley face as there is in the jackboot.

As an atheist, it seems to me that saying, "Christmas is really about *this* and Jesus would approve," is arrogating the power of religion for one's own beliefs, putting your own words into Jesus' mouth, even if it's to get Jesus to endorse viewpoints I agree with.  

I think it's much more honest to just say, "this act is morally bad/wrong because...," or "this act is morally good/right because...."  All else is a bullshit wrapper.  

(At least, according to my own internally-defined sense of authenticity.  It's not restricted to religious people, you know!)