I think people who do this simply aren't very familiar with the "rules of the road." I put that term in quotes mainly because there really is not a law that says faster moving traffic belongs, necessarily, in the far left lane. The far left lane IS a passing lane. But it is not a SPEEDING lane. Now, I'm coming at this, mind you, as a guy who rarely follows the speed limit. Under the law, if you are traveling in the far left lane at the speed limit, you are obeying the law. Typically it is the speeder who gets caught up in the far left lane by someone traveling at the speed limit, and thus the person in the far left lane is not driving slow, but driving at speed.
It is a matter of courtesy, however, that if you ARE traveling at speed in the far left lane, and recognize that another vehicle behind also in the far left lane is coming up on you in what is obviously a higher rate of speed, you should change lanes to let the vehicle pass.