It seems that there are just too many drivers that don't fully grasp the meaning of this sign. Semantically speaking, the two most important aspects of the sign are the comparative inflectional morpheme -er and the word right. I think it is a fair assumption that anyone who has a driver license knows right from left so the problem must lie with the comparative inflectional morpheme.
Following is an example that illustrates the semantics of slower traffic.
If you are driving 80 mph in the left lane on the expressway it is true that you are not driving slowly; however, if a car comes up behind you going 90 mph, by comparison you are clearly the slower traffic.
Additionally, it is not the job of individual drivers to enforce speed limits by blocking passing lanes, it is the job of law enforcement. It is always safest for drivers to remain in the right lane unless passing slower traffic.
Of course there are also drivers who fully understand the meaning of the sign but choose to ignore it. slowertraffickeepright.com does an outstanding job of explaining in more detail the safety and traffic flow benefits of staying right. It should also be noted that it is a law in many states to stay right.