The reasons are historical, not logical. And since religion is involved, and politics, for that matter, logic doesn't seem to apply.
If politicians can't do anything right anywhere else, don't expect perfection here. I, of course, agree that they should not be tax exempt. But if my opinion couldn't keep us out of Iraq, stop the NSA debacle or get tea partiers tossed into the ocean, I don't think it is going to work here either.