It really is simple: separate religion and government
The Court Affirms Separation of Church and State ... - New York Times: "The legal case against the displays is straightforward. The Ten Commandments are religious scripture taken directly from the Bible. The First Amendment requires the government to be neutral about religion, and putting the Ten Commandments on government property violates that principle. Installing them in a courthouse is especially disturbing, because it sends a message that the legal system will favor the tenets, and perhaps the adherents, of a particular religious tradition."