Yeah, I think it's instinctive. In the wild, the mother would eat the pups droppings to prevent predators tracking them apparently. I have heard that a way to stop it is to add pineapple to their food as it makes it taste no different on in way in, but (even more) unpleasant when it comes back out...
Its not normal but its not uncommon! You can get some chemical stuff that stops them, but as said above, pineapple is supposed to work.
Managing them is the best way, try to be there when they poo and beat them to it, or be ready with some nasty spray (not toxic though) to cover it before they can eat it. If they eat other animal droppings on walks try using a muzzle while out.