The "warning" that is so brutal it forces everyone else into obedience is one of those ideas that keeps coming back generation after generation, even though we can plainly see that it didn't work for any of those previous generations. (A little like the "short, victorious war".) Or, to quote Citizen G'kar, "it didn't work then, and it didn't work later".
@Zinnia: If you're curious about how much damage it could actually cause:
A single US Ohio-class ballistic missile submarine carries up to 24 UGM-133 Trident ballistic missiles. Tridents are fitted with submunitions called MIRVs that allow them to launch warheads at separate targets, or separate points around the same target. Each Trident has 8 or 12 warheads, depending on whether or not you think the US is following its strategic arms limitation treaties.
The warheads on these missiles are thought to be W88 thermonuclear bombs, with a yield of about 475kt (equivalent to 475,000 tons of TNT). Judging nuclear blast radii is a tricky business, but it would be reasonable to expect wood or brick buildings to be obliterated out to about 5 kilometers, and the thermal pulse of the explosion can cause fatal burns and start fires at easily twice that distance. (It should be noted at this point that the actual area affected has a lot to do with the topography, the type of construction, and environmental factors; this is a very rough estimate.)
The area of a 10km-radius circle is a hair over 314 square kilometers. If we assume they're not overlapping (SOP would actually be to overlap the warheads around a central target, but that would lead to more destruction where "damaged but survivable" areas of each warhead overlapped), there could be up to 192 of them per submarine, for a total of around 60,000 square kilometers of destruction - or to put it another way, 77 New York Cities (not counting the water). (Or 90,000 square kilometers, if we've been ignoring START.)
Obviously it's not going to kill every Muslim on Earth, but targeting the, say, 20 largest Muslim-population cities could definitely knock it down a few places on the list of religions by adherents.
So let's hope that someone more rational is involved in the process than the average Youtube commenter...