The exemption of state and local government employees from mandatory federal OSHA compliance was due to arguments supported by state and local governments concerning the application of unfunded mandates by the federal government on states. Underlying such arguments were the Tenth Amendment and differing conceptions of federalism.
Individual decisions by states, perhaps, can be attributed to local ideas of community choice as described by Charles Tiebout ( see http://www.csiss.org/classics/content/43 ).