let's say that we have a certain game animal in the area. the local hunting laws restrict harvest to only mature males, making females and immature males illegal to hunt. as a result over generations the female population rises and natural selection takes hold and since all of the healthy, larger males are being harvested while the smaller sickly ones are left in the gene pool all of the male offspring are also smaller and harder to distinguish from juveniles. with none of these animals being harvested the population explodes and the animals begin to starve. the local authorities refuse to bring the population down to healthy numbers and refuse to lighten the restrictions on harvest requirements, meanwhile the animals suffer.
Regardless of the illegal aspect of poaching in the "hypothetical' query which as Art has pointed out is not a new query/argument, what I find most troubling is the clear lack of understanding of what is going on or the overly creative justification for killing. Either way, it is pretty ludicrous.
If natural selection is taking over (which is a blatantly stupid notion since hunters have culled the population heavily and so isn't natural and in response to whatever impacts that there are on the population, natural selection is always in operation amongst the remaining population), why would you think you need to have hunters go in and cull the population? What is it that you think "natural selection" means? So the animals are starving for a few years. Yes, it is a horrible tragedy, but you know what, natural selection will take care of that if allowed to proceed. Going in to artificially cull the population isn't letting natural selection do its job just like the over cull of large males didn't.
Natural selection is not a kind process. It never has been. Nature is not kind at all. Every few years we see jackrabbit and prairie dog population explosions followed by population crashes that happen quite naturally, but nobody is worried about the jackrabbits and prairie dogs. So long as there is habitat, the population will adjust accordingly.
The notion that hunters taking all the big healthy males has left a population of smaller sickly individuals is naive at best. The trend may be for a reduction in the size of the individuals within the population which is a perfectly natural result, but being smaller does not indicate that they are sickly. In fact, the smaller individuals may be much more healthy. If the population explodes relative to the food supply, as with populations of large animals that end up on small island such that over time, the species actually reduces in overall size on that island in a process called insular dwarfism. The classic example of insular dwarfism that has been repeated in different parts of the world is with the dwarf mammoth elephants (rather silly terminology like jumbo shrimp).
In short, when a selective process acts on a population, the population will respond naturally. So when large males are selected against, in this case by hunters, the population will respond naturally. The overall size of individuals in the population may be reduced, but that is not an indication of poor health. So ethically, it would be wrong to go out and to try to kill off a bunch of this alledged smaller sickly population because you are not letting natural selection respond appropriately. All it really sounds like is a way to come up with a faux scientific justification to go out and shoot animals in the name of helping them. That doesn't wash. Also, human hunters being what they are, you can bet that the hunters are not going to just cull the "sickly" undersized adults. As already noted, males are hard to distinguish from females and and both from juveniles. So what your hunters are going to likely do is end up shooting the biggest individuals they see, further damaging the potrential for the desired larger sized individuals.