The "absolute immunity" for prosecutors was established in Imbler v. Pachtman (1976). The logic is that if prosecutors could be sued by people they accused but

The "absolute immunity" for prosecutors was established in Imbler v. Pachtman (1976). The logic is that if prosecutors could be sued by people they accused but failed to convict, that "would cause a deflection of the prosecutor's energies from his public duties, and the possibility that he would shade his decisions instead of exercising the independence of judgment required by his public trust." (http://scholar.google.com/scholar_case?q=Imbler+v.+Pachtman&hl=en&as_sdt=2,14&case=5758861728040203406&scilh=0 http://scholar.google.com/scholar_case?q=Imbler+v.+Pachtman&...)