Giving away food could impact sales if the very same people would be people that could buy from the same stores. A food bank is a supplement and rarely the only source of food for those people. If there was some sort of tax on wasting food, i.e. processing food waste then you can be sure they would be giving it away if that cost them nothing. However it might impact the selling of fresh food if shops were punished for waste and commercially they would be better off selling processed long life food.