The law they use is impeding traffic.  If you are just ignoring a growing line of vehicles behind you, driving well under the posted speed limit, and there is no reasonable explanation for your doing so and not pulling over when possible to let folks pass - then you can get a ticket for impeding traffic.
The OP's observation was a group of young folks purposely impeding traffic and they got ticketed for it - that is exactly what should happen.
Driving a motorhome or towing a trailer and not being able to reasonably pull over for a few vehicles is a totally different beast.  You are unlikely to get ticketed because you are not purposely impeding traffic.  
Once you have 10 or more vehicles behind you and are ignoring turnouts that you can easily turn into, then you have past beyond "reasonable" and into "impeding traffic".
Common sense prevails:
Forcing others to go slow because you don't think they should drive that fast or because you don't think you should have to move over is impeding traffic.
Not being able to safely pull over and driving slowly due to the vehicle size, capabilities, etc., is not impeding traffic.