Not true they can be fired. The state department for one! Richard Clark for another.
As someone who has worked for the USG for 36 years, including 28 in the State Department, you are totally and absolutely wrong. Clarke was a career employee in a Schedule C position. He could and was replaced by Condi, but he could not be fired without cause. The Plum book has a list of all the political appointee jobs in the government.