Since the days of the earliest colonists, a majority of Americans have belonged to Christian churches or have at least expressed a preference for Christianity. In consequence, Christian symbols such as the cross and nativity scenes have often been displayed on public property. For centuries, the small minority of non-Christians, including Jews, adherents of other religions, and believers in none, didn't complain much about this, even if they were offended. If they did object, they were largely ignored. In recent decades, however, as the United States has grown increasingly diverse, with millions of adherents of other religions and large numbers of people who subscribe to no religion at all, objections have grown.
The objectors have contended that when a state or city displays a cross, nativity scene, or other Christian symbol on public property, it implies official governmental approval of Christianity, relegating their religion or non-religion to inferior status. They, along with many Christians, believe this runs counter to the Constitution, which establishes a secular state that allows the free exercise of religion but places its stamp of approval on none. (For my position on whether the United States is a Christian Nation, please see the Opposing View debate at http://www.opposingviews.com/questions/is-the-us-a-christian-nation).
Some of the time, voicing an objection has been enough to persuade public officials either to stop displaying the Christian symbols or to allow the symbols of other religions and cultural stances to be displayed at the same venues. When that has failed, those offended and other interested parties concerned to preserve the separation of church and state have taken their concerns to court. Repeatedly, though not invariably, the courts have sided with the objectors.