Well, the ones that do, are risking lawsuits. I can say that Bill Nesbitt’s version is different than the actual Scruggs version; I’m not sure if that has any impact on the situation.
It comes down to the question, does an artist have the rights to his own work and is he entitled to keep earning an income on his own creations? I think yes. Even though Earl has passed, his body of work is still owned by his estate, his family. Are they entitled to protect that property? Sure, why not?
To clarify though, I think I’m correct in saying that @BanjoBen’s limitation is in “teaching” the song. There are plenty of tabs available from various sources and you certainly could ask Ben if he has a particular tab and if he could send it to you. Someone correct me if I’m wrong.