True Vegans do not drink cows milk because the cow milk is derived from a cow.
True vegans also avoid any dairy products whatsoever and not just cows milk.
There are some Vegans however that do drink cows milk but those wouldn't be true Vegans in my opinion.
But there's a name for those type of Vegans and they're called lacto-ovo Vegetarians and those Vegans eat Dairy, drink cows milk and even eat eggs.