I disagree.
I can understand taxing churches (and as a churchgoer, I would regret the hurt it would do to many of them) - and I think it will eventually happen.
But I don't think you can do it by judging the content of sermons. I think it should be all or nothing.
Who is going to judge if a sermon includes political messages? And is it only sermons? How about during announcements? Only the pastor? How about the church secretary? Are we going to send government spies to church to rout them out, or rely upon parishioners to "rat out" their pastor?
Too many problems. All or nothing.