What part of the constitution says employees are anything more than at-will?
First amendment, extended by the fourteenth, says that a State cannot abridge free speech, religion, etc. . You can't fire a state employee, therefore, for something he/she says away from the workplace, or for his/her religion. A private employer, constitutionally, could fire someone for saying something the employer didn't like, or for his religion. 14th amendement says the state must give citizens equal protection of the law; again, this prevents 'at will' firing. The Bill of Rights also requires due process (which goes back to the Magna Carta: see http://caselaw.lp.findlaw.com/data/constitution/amendment05/11.html)