Free speech is a testy topic, and the deal - draconian perhaps - to trade off free speech for a tax exemption seems trivial until you consider that the US church is probably the largest land holder in the nation behind the government.  To risk the property tax exemption so Fr. Wise, Unwise, or Otherwise can opine about a particular candidate from the pulpit , risks throwing out the baby with the bath water.  
Besides, if a priest hits your stated hot buttons of abortion and homosexuality from the altar, he is fair game for someone in the congregation to stand up and hit back.  The altar may be elevated, but the high ground isn't, and while Catholic moral theology is foundational, moral/political opinions are just that, an opinion, and everyone has them.