"Dixie County is not establishing a religion by allowing a private company to place a monument in a location where similarly donated monuments may be placed," said Matt Staver, founder of Liberty Counsel. "The Ten Commandments are universally recognized as symbolic of the law, and are appropriate for display in courthouses and similar settings. Public display of the Commandments is consistent with our nation's history and with the First Amendment. There are more than 50 depictions of the Ten Commandments in the U.S. Supreme Court, and there have been thousands of displays throughout the country for many years."
Source: Florida Baptist Witness
There are few legitimate defenses for having a Ten Commandments monument and it's clear that Matt Staver, whose Liberty Counsel will represent Dixie County for free, is choosing one of them: in contexts where various messages are permitted, the government cannot keep out one message simply because it is religious. In general, this is a fair argument. So long as the government has created an open forum where there is no content-based restrictions on the messages which private citizens express, the government cannot arbitrarily shut out religious content and religious messages.
The question which the court will have to answer, then, is whether the location of this Ten Commandments monument really is any sort of public forum. Is there a history of the county government allowing private groups to erect permanent monuments or structures like this in this location? I rather doubt it, which would suggest that this claim is only being made retroactively in order to justify what was done — and courts tend to take a dim view of such behavior.
What about going forward? Can the county government and the Liberty Counsel argue that this location is now a "public forum" where private speech is permitted? The would be easy enough to test: a secular, atheist, or humanist group can petition to erect their own monument right next to this one. It should be about the same shape and size but carry a message like "The United States is not in any sense founded on the Christian religion" or "No Gods, No Masters."
If the county government rejects the proposal, that will suggest they are playing favorites and the location is not really a public forum. If the county government accepts the secularist monument, this will outrage those who regard Ten Commandments monuments as a medium for promoting their religion and may thus dissuade them from using arguments like those described above by Matt Staver.
Heather Wellman, Executive Director of the Humanists of Florida Association explains why Ten Commandments monuments like those in Dixie County are so wrong:
In order to walk into the Dixie County Courthouse, one must walk past the large Ten Commandments display. Each person entering the Dixie County Courthouse is confronted with the message that the Ten Commandments are central to the beliefs of those running to the courthouse and that God, not justice will be the ultimate guiding force while in the courthouse. This messaging is an affront to the democratic ideas of the American justice system and sends the message that the tenets and laws laid out in biblical texts can and will usurp the laws established in America.
Even more important than the message the monument sets is the precedent it lays for future action. Imagine prayers before sentencing or collection plates in traffic court. It may seem unfathomable that the United States would adopt a state sanctioned religion, but it is only so unimaginable because the separation of church and state has been diligently protected against small and large infractions.
The citizens of Dixie County Florida deserve to go to a courthouse without the implied endorsement of Judeo-Christian religions. In Floridian courtrooms, the law and justice, not religion, should reign supreme. Each person, regardless of religious preference, should feel comfortable entering the courthouse and should feel as though they will be treated fairly regardless of their faith or lack thereof.
Wellman makes a lot of important points here. Why must people entering a public courthouse on public business be confronted with a message that a particular god is favored by the government? This arguably conflicts with atheists most of all, since they don't believe in any gods, but it also causes problems for various theists as well because not every theist believes in the god associated with the Ten Commandments. Neither atheists nor theists need to be told that the "justice" they will receive is in any way connected to the Ten Commandments.
The problem with "precedent" is also worth considering. People already try to make arguments for greater government endorsement of Christianity and theism on the basis of things like "In God We Trust" on our money, so how much further will they try to take things on the basis of the Ten Commandments? If local governments are given the authority to promote and endorse the Ten Commandments, why not Christianity generally or the resurrection of Jesus in particular?