Because American culture has been dominated for so long by Christianity and Christian traditions, Christmas is celebrated widely even by non-Christians. Indeed, it's getting to the point where Christmas is becoming less Christian in nature every year. Nevertheless, many non-Christians continue to perceive the day as fundamentally Christian and so don't want to celebrate it. Because of this, there are growing numbers of non-Christians who refuse to have anything to do with the holiday, even in a secular nature. They don't exchange gifts, they don't put up decorations, and they even go to work if it's a normal work day.
There are good arguments for both sides of the issue. The Christian character of Christmas is diminishing over time and the ancient pagan aspects are taking on ever greater importance. Christians don't consider themselves pagans, yet have no problem putting up a tree and mistletoe. Atheists and humanists don't consider themselves pagan and also don't mind putting up a tree or mistletoe. If a few Christian aspects of the day are included, does it really matter very much?
Perhaps it does, or at least it can for some people. It's reasonable to argue that celebrating Christmas helps further the idea that America is a Christian Nation. At the same time, though, does not celebrating it do more to undermine this claim than celebrating it in an openly secular, non-Christian manner? If the decision between celebrating vs. not celebrating is to turn on political considerations, there's a lot to be said for helping along the process of secularization and de-Christianization.
Of course, it's also legitimate to question whether this should be a political question at all. Not celebrating Christmas because it's not part of your religion and/or because you don't care about it makes more sense, to me at least, than doing so in order to make a political statement. Christians have begun to use Christmas to make political statements; humanists and atheists perhaps shouldn't imitate them. If they do, it should probably be to make a point of rejecting the imposition of Christianity on the culture.
The above image is based on a World War II poster depicting Uncle Sam rolling up his sleeve and going to work for the sake of the war effort. Here he's getting to work while ignoring Christmas because corporation all need us to work long hours.