Is there anything dangerous about Christmas? Perhaps. Most Christians and even most non-Christians regard Christmas as being at least innocuous if not a positive good. This is not a universally shared opinion, though. There are people who are critical of Christmas, arguing that it has more negative aspects than positive ones -- or even that there's nothing especially positive about it at all.
For some Christians, the Christmas holidays represent remnants of ancient paganism which should never have been incorporated into Christianity in the first place. Their presence are a sign of the incomplete work that still faces true Christians today. People who observe Christmas holidays aren't following Christian holidays but are, instead, simply helping the dark forces of paganism.
For many non-Christians, the Christmas holidays represent the dominance of Christianity in America. They may be appropriate for Christians, but they are wholly inappropriate for non-Christians. To observe them would be to give into the cultural forces behind Christianity and the tendency for American Christianity to assimilate everything it encounters. Resisting this is necessary for the preservation of ones own cultural and religious traditions.
Many irreligious atheists have a perspective similar to that of non-Christians. Although they aren't likely concerned about preserving their own cultural or religious traditions, they don't want to become assimilated by Christmas and Christianity themselves. Although Christmas is less exclusively Christian than it used to be, they still perceive it as predominantly Christian in nature and, since they aren't Christian themselves, they object to being expected to observe it in any fashion simply because it's popular.
These are all completely legitimate objections to Christmas and the insistence that everyone observe Christmas together. There is no question but that Christmas has developed a tremendous amount of cultural momentum in America. Avoiding it can be difficult at best -- I'm not sure how anyone can be completely successful. I can certainly understand why they would try, though, and their insights into the possible dangers that exist in Christmas should be taken seriously by everyone else.
This image is based on a World War I poster encouraging people to be patriotic.