boneshaker, I think the point of riding MLK is because they can. Kind of a low-key, Critical Mass sort of thing, hopefully obeying traffic laws along the way-- Right, Elly???
Probably for some people, MLK is the most direct route to their destination. Why should they ride a few blocks out of their way? You wouldn't suggest that drivers do that, would you? Riding down MLK like this points that out, I guess.
Personally, I wouldn't ride down MLK... but then again, I don't have any reason to ride, or drive, down MLK.