Except that ruminants are able to digest waste products that would otherwise be, well, waste, and live on land that can't be farmed for other crops. The animal feed argument is much more relevant to pig and poultry farming, where they eat a relatively high quality diet.
Dairy cows in this country, and most of europe, eat grass, silage, brewers grains, beet pulp, citrus waste (what's left after making juice) and all kinds of dodgy by-products.