Well, think about it this way. Why are pedestrians banned from walking on roads, even when there are no nearby sidewalks? The answer, I believe, is that it's extremely dangerous, both to the pedestrian and to automobile drivers, and that it would cause massive inefficiency in the flow of traffic. The reality is that bicycles, like pedestrians, necessarily behave very different than automobiles, and most of our roads and traffic laws are designed around automobiles. Similarly, bicycles are not allowed on certain types of roads (many bridges, freeways, etc.) Obviously, its a continuum, and bicycles can fit in with automobiles better than pedestrians. The question is simply where to draw the line.