Similar issues are raised when a state bar decides to make all admitted lawyers perform x number of hours of public interest law. It sounded like a good idea to me, esp. when you work with low-income people. The prof asked us if we really wanted ultra conservative lawyers to take time off from making money to advocate for their wacko causes. No, they don't need any requirement. Columbia Law made every new law student work for one years after graduation representing poor people.
I doubt if Nixon was the first to use the tax code to punish detractors. Some Obama appointees were doing the same. Taxpayers could use a break. There would have to be no personal judgments in revoking tax exempt status. I wonder if citizens argued against tax exempt status for churches when the issue first arose. One thing that is important to remember is that churches were the only deliverer of social services until FDR's New Deal.