You are right that cattle forage with other species in Africa and North America. However, if you went to a zoo that had a display of elk, pronghorn, or bison, would you really want to see a herd of black Angus cattle with them? To me it wouldn't be the same as having the elk, pronghorn, or bison by themselves, it wouldn't look as authentic. This creates a sort of double standard as I do have to admit I found the exotic Ankole cattle to be very impressive animals (I took plenty of pictures at Disney's Animal Kingdom). Since they are an exotic breed that is probably why most of the visitors probably overlooked the fact that they were just cattle. Even though I took pictures I still kept thinking that they were cattle and not wild animals and it did tend to take away a little of the experience. I can also see how this could be a strong message talking about forage competition though.