Traveling slowly^ in the left lane both causes congestion and creates a situation where passing on the right is likely to occur. Both of these dramatically increase the likelyhood of an accident. The slow left-lane driver is at fault for not following the otherwise accepted conventions of the road.

Laws and legal speed limits have nothing to do with it. (however: in many areas the person traveling at slow 'regular' speed in the left lane would be breaking the law. In many areas the left lane is explicitly for people traveling faster than the speed limit (read: passing)).

^where slowly is defined as relative to the rest of traffic.