The group is referred to as a "rolling roadblock".
Of course that implicitly distributes the blame, where some would rather direct their ire to the schmuck in the left lane who isn't bothering to pass.
However, it should be noted that sometimes this is done on purpose. For instance, trucks (who are often in communications with each other via citizen band radios) will occasionally do this when approaching a speed trap, so that no poor unsuspecting soul will get caught.
Also note that in some states (I know Kansas used to be this way) you can get a ticket for doing this. "Traveling in the passing lane" or somesuch. A cynic might postulate this is more so that they have a reason to ticket folks who are preventing their speed traps from working than it is for public safety. But where would we find a cynic on the internet?