As I've stated before, I think PETA is kind of a nutcase organization though I *am* vegan myself, for health reasons. My cholesterol is 120 or less every time I get it measured and I'd prefer to live a life free from heart disease and having to take cholesterol lowering drugs.
That having been said, people do MANY things for THOUSANDS of years that are bad for them. Like, drinking alcohol and smoking tobacco. These things cause more deaths combined than every other illegal drug out there, but people still keep doing them - even I enjoy a few beers on occasion.
Just because people have been drinking milk for thousands of years doesn't mean it's good for you. Though, every individual's biochemistry is different. Asians, for example, are mostly lactose intolerant and can't drink milk - they lack enzymes to digest it. (You don't find much dairy products in traditional Japanese diets, and the Japanese are some of the healthiest people in the world - at least, the ones eating traditionally are.)
Milk has actually been linked to causing type 1 diabetes and a host of other health problems; I certainly wouldn't feed it to my kids.
But, it has nothing to do with animal cruelty. I don't view eating animals as cruel - I have five cats; they all eat meat because cats need meat. Humans do not.