I think it is important to mention the extent to which the conditions (rules to exercise the privilege) of government employment go. For two and half centuries of Common Law one of those conditions has been that the exercise of government privilege of whatever sort or fashion (employment, officeholding, contracting, etc.) can be taxed. This privilege or excise tax is generally always measured by the gain, profit or income that the privilege produces. Blackstone’s ninth excise and Sections 86 and 90 of the 1862 internal revenue act are excellent examples of this privilege tax or excise. Most of the States utilize this tax via their personal income tax acts, as do the Feds, since 1862.