Now, let me begin by saying that in '62, everybody knew that the only good shark was a dead shark, and we were just cleaning up the local beaches and keeping 'em safe for swimming & surfing. Now we know better.Genesis once bubbled...
The other side of this is that if you powerhead the shark there is no guarantee you will kill him (especially if its a large shark), and you've just dumped a LOT of blood and guts in the water, which is not all that good if there is more than one shark around!
Unless something's changed about sharks or powerheads in the last 40 years, neither conjecture above is true. (of course our homemade devices used 12 Ga shotgun shells rather than .38 cal pistol cartridges) I can state with absolute certainty that any fish up to 300 pounds hit anywhere from the middle of the head to the middle of the body with a 12 Ga powerhead will just roll over dead, and there won't be much in the way of blood - and no guts - in the water. All the powerhead material is stopped in just a few inches of fish flesh (as there is no "barrel" per se on a powerhead, the projectile doesn't get moving much at all before all the gasses propelling it are dispersed into the surrounding flesh, so there is very little penetration) - the shock and damage from expanding gasses are so severe that death is instantaneous, including the heart, and the blood stops flowing immediately. The wound will "leak" a little, but won't flow like one might expect. The shock also turns a lot of fish flesh into useless mush that's inedible. A horribly wasteful way to harvest fish.
As Walter has said, powerheads are not efficient for harvesting, and nobody that I know of uses 'em for that.
There is no compelling reason to get the government involved, and lots of compelling reasons to keep the government out of Scuba.
Rick