The Holiday Tree
Do any of these other religions have such trees? If not, does rhetorically forcing them to note this one under the banner of "holiday tree" not constitute a greater offense than the state simply having a "Christmas tree?" I can see a valid constitutional argument over whether government should decorate for religious holidays, but since they do it makes sense for each religion to retain its own symbolism rather than try to generalize everything to everyone. You can treat it as a learning experience in the area of cultural diversity.
Worth noting is that although only 80% of Americans are Christian, I recently saw a stat on a TV network (FOXNews, I think) indicating that 95% of Americans celebrated Christmas. It's really not unusual for a holiday with religious origins to develop a more generalized cultural identity. I find it amusing that Santa Claus is often listed as a secular aspect of Christmas when he is, in fact, derived from a religious figure.