You are right. They made a law that is unconstitutional. Either they apply taxes straight across the board and not select any preacher out because he said something that falls in the grey area of maybe that's political.
Fact is nearly everything that a preacher may preach on would have something to do with morals. Morals such as "homosexuality is an abomination to God". Or how about in Genesis where it says that "he shall rule over thee", something that would freak nearly every woman in this country out today. Woman's lib stuff.
Both item are today hyper political yet preachers probably risk answering a call from the IRS.
What I am getting at is the law is unconstitutional because it singles people out for exercising two of their enumerated rights in the constitution. Freedom of speech and freedom of religion.
They should either tax them or not, and get the hell out of judging whether what a person says behind the pulpit or not is legal . It is also punitive in that it punishes people for what they say.