They should be pulling over black vans and brown ones too! I mean, it's just common sense, right? As soon as the sniper knows everyone is looking for a white van, he's going to start using a different type of vehicle. The cops will be looking for a white van while he cruises by in a green Saturn.
That's analogous to the argument I've seen against profiling at airports. As soon as we start profiling Middle Eastern men, they're going to start using 80 yr-old black women to hijack the planes...