He has the right to preach Sharia law if he desires. Now, with that said, where is the line? Would he be allowed to preach that people should commit murder? What if a private citizen told his neighbor that the neighbor should spank his child? Can he be sent to prison? Very slippery slope here.
Or what if the preacher in the course of his weekly bible readings happens to recite that chapter in Leviticus where it says Homosexuals should be put to death. Then a while later someone in the congregation with a few screws loose kills a homosexual. Under this precedent, the preacher could be locked up for doing nothing but reading a verse as it is written in the Bible. A very slippery slope indeed...