There are, I believe, three reasons why putting a Star of David on a Christmas tree is wrong:
(1) It is offensive to Jews. The history of Christianity is largely supersessionist. For two millennia, its dominant theologies have held that its purpose is to replace (supersede) Judaism. Jesus came to save the Jews, the theologies claim, and in return, Jews should all become Christian. Putting a Star of David on a Christmas tree reinforces this notion. It tells Jews that their only purpose is to convert and become something they are not.
Some might object, claiming instead that people put Stars of David on Christmas trees to celebrate diversity, stores and businesses especially. But all this does is suggest that Jews shouldn’t be welcomed as themselves, that they can only be seen through a Christian lens. Putting a Star of David on a Christmas tree tells Jews that yes, someone will begrudgingly deign to acknowledge them, but they will not admit that Jews are actually real people with a real religion. At best, butting a Star of David on a Christmas tree is lazy, at worst, it is a form of shouting “Are you happy now?!?! Get off my back!” If you want to welcome Jews into your stores, put up a menorah, and if you have a mixed family with people who subscribe to different religions, each member is important enough to warrant his or her own decoration.
(2) It is offensive to Christians. A central tenet of Judaism is that the Messiah has not yet come. This means that as far as Jews are concerned, Jesus was not the Messiah, nor was he the son of God. He is not returning, Jews believe, because he was never here. As a result, if you put a Star of David on a Christmas tree, you are, in fact, denying the divinity of the very figure you claim to celebrate. You are saying, in essence, “Merry Christmas…you know Jesus isn’t real, right?” That’s a pretty awful thing to say to people of faith.
My students sometimes don’t believe that Jews deny the divinity of Jesus, asking, in one form or another, “do Jews really not believe in Jesus?” They really don’t. Some Jews do offer compromises, suggesting that Jesus was a real person, but not the son of God; a prophet, not the Messiah (Islam argues this); or that he was a great rabbi, philosopher, or wonderful role model, but no actual Jew believes he was divine. Each of these possibilities are interesting in-themselves and I recommend taking some religion courses at The University of North Dakota to examine them. Nevertheless, they are all forms of denying the reason for Christmas and they have no place on a Christmas tree.
(3) It makes you look like a moron. It is always possible that the people decorating the tree simply don’t know better. They may not know what the Star of David is or what it stands for. If they are seven-years old, then they can be excused for not knowing and their mistake can become a great teaching moment. If however, they are in high school or older (as anyone who has a job will likely be), and they do not know what a Star of David is or what Jews believe, then something has gone fundamentally wrong with their education. Intervention is required. We live in a pluralistic society, and we inhabit a world with complex and interesting histories. Everyone should know the basic of all the major religions, even if they don’t believe in any of them. Further, if the tree decorator did know what the Star of David is and simply didn’t put two and two together, then this reveals a disturbing lack of thoughtfulness that also ought to be addressed. The world is a fascinating place, but one has to notice it before it becomes interesting.
There will, no doubt, be people who respond to this post by mentioning Jewish friends who grew up with Chanukah bushes: shorter trees that some families decorate to help their kids get through the anxiety and exclusion that comes from being surrounded by Christmas. (This has come to be known as The December Dilemma.) This is a controversial practice that had its heyday in the late 1970's and early 1980s, and while it is understandable that some beleaguered Jewish parents resort to it because simply because it’s easier, it does more harm than good. It teaches kids that there is no fun in their own tradition and that other people—the Christians, specifically—have it better. Suck it up. No one said parenting would be easy. Now take the Christmas tree out of your house.
Finally, it is certainly possible that there are people who want to put a Star of David on a Christmas tree because either they don’t care about offending people, or because they want to make trouble. There may also be people who are simply too lazy to bother with propriety or other people’s feelings. To them I can say only this: you are certainly within your rights to have your tree decorated any way you’d like, but don’t be surprised when it causes others to think poorly of you. In order to be respected, you have to respect others. A person’s religion, or their atheism, is a good place to start.