I agree. Freedom of association should be treated as carefully as freedom of religion, and all sorts of clubs and groups and associations get taxed up the wazoo, too.
The answer really is no corporate tax at all, profit or ‘nonprofit’. Let those associations be unencumbered, and then impose equal, low taxes on use of public resources and on personal income for all.
Admit it, the federal income tax is just another failed government program. Rather than tinker around the margins we should rescind the 16th amendment.