Well the Walker's may not be poisoning the pigs, but the pigs have all that putrid flesh and bacteria within them from the Walkers.
Thus the pigs may carry some type of disease or ailment that could sicken whomever eats them.
Also there is the question about the you win but still lose note that was with the supplies Rick and Aaron found. Is that food poisoned?
That would definitely count as trying to poison the food supply. Pigs would be a good way to pass along disease. It has never been substantiated that cooking will stop spreading the contagion. I will be surprised if this does not affect the saviors to some degree.