While I agree to the fact that there is a shortage of piggies in some areas, I find that donating piggies to the Giants helps to gain a lot of favor. Now, because I learned Animal Husbandry, I am willing to raise one pig per every one that I sacrifice, so as not to cause a depletion.
I would like for the ability to donate them to return. If the problem is a shortage, then perhaps have people donate their own piggies, rather than those on the street, or perhaps use some other method of keeping track of low pig numbers.