You are also to know that you have no authority to impose taxes, tribute or duty on any of the priests, Levites, musicians, gatekeepers, temple servants or other workers at this house of God. Ezra 7:24

            It’s always been a way to show their value to society by exempting them from taxes.  If a government taxes religious institutions, it also has a way of controlling them by overtaxing them and putting them out of business.  In addition, religious leaders make very little income (off contributions and offerings) so they always need a little help.