They should - in their own country. But Singapore had some very bad experiences with foreigners who try to make trouble in the name of religion. Last month, it was Moslems from across the causeway (surprise, surprise). A couple of years ago, it was a Catholic priest from the US getting air time denouncing our telephone dating services. Before that, it was the Jehovah's Witnesses with their familiar "push technology".
In a country with no majority religion, a certain reticence is adviseable - and, if necessary, enforced.