I have not seen anyone offend by Christmas trees, but I have seen them offended by the lack of Christmas Trees. Christmas is so commercialized now anyways that it is just a marketing ploy by companies instead of a time to celebrate Christ’s birth.
The Christmas tree wasn't even an invention of early Christians. It was an invention of the Druid tribe, as was the burning yule log. The Druids believed that the sun was burning out and only elaborate ceremonies including firing flaming arrows at the sun, burning yule logs and decorating evergreens would appease the gods and prevent the world from sinking into eternal night. The event climaxed on December 25, when the small increase in daylight hours first became noticeable.
In order to convert the Druids and other Germanic tribes to Christianity, it was decided to move Christmas to December 25 in the waning days of the Roman Empire.
At one time, this was such common knowledge that the Puritans and the most anti-Catholic zealots among the Protestant world banned the celebration of Christmas, including Christmas trees. It was only by the later half of the 18th century as the Puritans mellowed and settlers from other groups (notably Quakers, Catholics, Germans, Swedes, Dutch and Jews) reached critical mass that the restrictions on Christmas celebrations were relaxed and the Christmas tree made a comeback.