I think it's worth noting this study isn't about what people think. They didn't ask them whether they were offended. They made them take an unrelated psychological test and the result was that they were insecure when a Xmas tree was in the same room.
But like I was saying, the root problem is Americans don't share holidays.
It's hard to tell without seeing the actual study, but I don't think that's it either. I suspect the study's conclusions, at least as reported by MSNBC, are backwards. It's not that non-celebrants feel more down and insecure when confronted with a Christmas decoration. Instead, I suspect what the study really shows is that those who do celebrate react positively when reminded of Christmas.
Their "scores" of celebrants are higher because they saw the tree and it made them a bit more cheerful than those who don't celebrate. I suspect you'd get similar results if half the group heard cheerful music and the other half didn't, or half were in a room with a picture of a favorite vacation spot, or hobby activity, or something similar that provokes good memories. In other words, rather than being a negative influence lowering scores, I suspect the tree was a positive influence raising scores of those who celebrate Christmas.
But, that's just speculation. There's far too little information in the story to reach solid conclusions. The sample size is so small it would be easy to taint with just a couple of outliers.