Duncan was a storyteller in the oral tradition. He would have been horrified to hear that some storytellers imagine that copyright applies to the oral tradition, or that they might avoid telling stories because he had published them in an effort to spread them around more! He didn't own his stories, and nor does anyone else. Publishing the words of one particular telling doesn't give rights over any other instance of its telling anyway, and not just when someone deliberately changes it. The oral tradition is just that, and books are merely a modern convenience on top of it.
All storytellers I've met, except for in the USA, pay no attention to copyright and can't understand why Americans are so obsessed with it - it's a much misused law brought in long after the oral tradition brought all our wonderful stories into being, and aimed at preserving printing rights for original work not at stealing stories from public ownership or telling. When storytellers in the UK hear about the US hand-wringing they literally look open-mouthed at each other and shake their heads - I've seen it many times! Duncan was merely a caretaker for many stories, and not the exclusive one. He could no more have asked tellers not to tell the stories he knew than he could have insisted they not breathe any of the air he had breathed. It would be like a priest teaching the wisdom of God, but then instructing everyone listening that they would have to go and invent their own god to worship because his was taken.
Of course Duncan, like all tellers from the oral tradition, forcefully insisted that anyone hearing 'his' stories had a duty to re-tell them. I can't emphasise this enough to Americans - being a storyteller in any traditional sense means that you have a duty to pass on the stories, not to tie yourself in knots about an irrelevant modern law that, if invented earlier and wrongly interpreted as in modern US telling circles, would have ensured we didn't have a body of wonderful tales in the first place. Stanley Robertson, another wonderful Scottish Traveller like Duncan, tells his audience 'Now you've heard my tale you MUST NOT rest until you have told it to someone else'. Does that sound like he might be concerned about someone 'taking' his story?
There is also another strong value of the oral tradition, not always followed now that fewer tellers have been steeped in it, that one should never change a traditional tale in its essential form. That means no combining stories or changing the ending etc. so that it becomes your 'own' version. If the current tactic of US tellers trying unnecessarily to be 'ethical' by introducing personal alterations were to have existed a millenium or two ago, we would not have the amazing body of long-lived tales today that storytelling depends on. Storytelling is based around communal values, not individualistic territorialism trying to ring-fence versions or avoid stepping on others' territorialism. Normal polite respect and professionalism goes without saying, but bowing before egoistic protectionism is totally destructive to the essence of the storytelling tradition.
That is the generous and communal attitude of the tradition. Now please go and tell stories, for they are more important and enduring than the mere storytellers, however skilled, who briefly catch the ball of thread of ancient tradition before tossing it on to the next weaver of words.
At Tim's suggestion, I should point out that his argument is rather condensed and comes after a long and ongoing debate in the storytelling world. This is neither a comprehensive answer to my question nor a definitive one, so consider any pronouncements here part of an exchange of ideas.
Tim Sheppard is a storyteller and coach. You can learn more about him online at his web site, http://www.timsheppard.co.uk/story/, and on Facebook at http://profile.to/timsheppard