What's up with Jews? Why do people treat Judaism like it's a race?

Every Jew I know of (Sammy Davis Jr. and Whoopie Goldberg aside) is white. What's up with people who say things like; 'I'm half Jewish'. What the hell difference does it make? Isn't Judaism a religion? Aren't most Jews of European descent? I don't hear anyone saying; "She's 1/4 Christian on her mothers' side." Someone explain.