As most of you already know, Islam is one of the major religions of the Earth, the Muslims constitute approximately one fifth of the world's population and they are scattered throughout the five continents. It would not be so if Islam would not be considered universal in its teachings and practices and expects adherence and obedience by each and every Muslim regardl ess of their nationality and history. But, because of its world presence, some habits and tendencies may differ from one area of the globe to another, as the Muslims become a part of that area or that country and, in some degrees, exchange information with the rest of the world. On the other hand (an explanation needed for the people who does not know the term) Haraam (or haram) is an Arabic term meaning sinful. In Islam it is used to refer to any act that displeases or angers Allah. Such acts are typically prohibited in religious texts and is the highest status of prohibition given to anything that would result in sin when a Muslim commits it.
That's why some people say that celebrating Christmas (in any way) would be like building your own place in hell - that usually happens where the Islam is the dominant religion. In other countries (immigrant) Muslim that celebrate Christmas are only admonished.
The clear the things out (even for the people outside Islam): in the Qur'an there are many stories about the life and teachings of Jesus Christ (known as 'Isa in Arabic). The Islamic holly book recalls his miraculous birth, his teachings, the miracles he performed by God's permission and acknowledge his life as a respected prophet of God. Also, Muslims believe in the virgin birth of Jesus, but most of them do not mark the occasion as a holiday: Islam is considered to be a complete way of life and it does not condone innovation (Bid’ah) in religion. From that point of view American Muslims (or any other Muslims over the globe) celebrating Christmas commercially or otherwise is contrary to Islam.
Islamic practices has absolutely no bearing on the status of Islam as a religion. There are only three occasions that are permissible which are specifically mentioned in Qur'an and in Hadith (narrations of Prophet Muhammad): Eid ul-Fitr (celebration at end of Ramadan), Eid ul-Adgha (celebration at the end of Hadj) and the days of Youm ul Jumua'ah (every Friday). As a whole, Muslims do not celebrate Christmas because Islam construes its celebration as haraam.
As you all noticed, Christmas has become a commercially beneficial holiday period for most (all over the world), but especially for those instrumental in pushing the sales of Christmas gifts, presents and trees, flashing lights, greeting cards, gift wrap, etc. Depending of the community and context, some Muslims have chosen to celebrate Christmas by offering practical gifts to people in need and writing cards with wishes of happy holliday, health and wellness.