Who says that the 10 Commandments constitute an establishment of religion? Are the people in the court house worshiping the 10 Commandments? Since one of the commandements is not to commit murder, is the law against murder a religion?
The Ten Commandments are to certain "Religions" as a Solid, Great Foundation is to a poorly constructed house. The foundations are essential and are fine. The people building on top of the foundations are the problem. They build dilapidated and flimsy houses that offer the occupants no peace, shelter or sanctuary.
Any reasonable person would conclude that an agent of the state, especially the punitive one, displaying one religious symbol over all others constitutes an endorsement of that religion. The law against murder is not religion, but is part of our legal code. We also have laws against theft. We do not have laws against adultery, lying (unless under oath) coveting, misusing God's name, etc. Should we display only those two commandments that are actually part of our legal code?