Up until very recently, people living in far northern climes didn't have the option of such a diet (and economically, they still don't... imagine having to fly in all your food to satisfy a moral argument). In the winter there is literally no vegetation. Would you argue that the Inuit should starve because we down here in our balmy regions disapprove of whaling? In their case, it's subsistence, not preference, and they use literally every part of the animal.
The Tibetans eat a lot of meat, because they too live where there is very, very little vegetation that humans can digest. And they don't have a reputation as being 'cruel'.