Why can't most Christians understand the need for separation of religion and govt?

[Mystical]

http://www.cnn.com/2015/07/02/politics/o...index.html

After the Supreme Court's week of high-profile decisions, Oklahoma's high court made a stir of its own in a ruling that had a group of self-proclaimed Satanists and an ordained Baptist minister celebrating.
The Oklahoma Supreme Court ruled Tuesday that the state must remove a 6-foot tall granite monument of the Ten Commandments from its capitol because it violates the state's constitutional ban against the use of public funds or property to benefit a religion.
The decision came after years of controversy and legal battles. The monument's supporters include prominent figures in the state, including Oklahoma Gov. Mary Fallin and members of the state's legislature.
In an interview with CNN affiliate KOCO, Bruce Prescott, an ordained Baptist minister and one of the plaintiffs in the suit against the display, said, "I'm not opposed to Ten Commandments monuments. I'm just opposed to the placement on government property, because I think that the government needs to be neutral."

Bravo to this guy. I'd vote him into public office even. 
Bold added by myself. When I was a Christian I understood the need for the separation. Why are sooooooo many theists in denial about this issue? Is it because they believe (their) gods law is above all law? If so, then would you Christians rather be put on trial with an openly biased Muslim, or would you want to be judged fairly and equally based on the facts at hand and judged accordingly with the laws set in place by a majority decision despite the religious implications of a book that has no relevance to government (ie: if you cheated on your spouse and a religious text dictates that you be stoned to death)?