There is some evidence that in times long past at least some of the 'song bearers' who transmitted traditional songs down the generations might also have doing it at least partly for money. (And anyway I'd suggest that Bob was almost certainly joking - you can't be that good and not also be passionate about your work)!

While we should always admire and respect the people from whom traditional songs were collected, they were only the 'bearers' of those songs at that point in history. Their personal philosophies re the 'ownership' of songs are a snapshot, at the date of collection: A key part of the story, but only a part.

Also, one might surmise that the way source singers spoke about their material might have been influenced by the fact that they were speaking to a collector, and aware that 'their' songs and versions - at that point known only to a few - where about to become much more widely available. This might have tended to concentrate their minds on the moral aspects of 'song ownership' - something that previous generations of singers might not have thought so deeply about - specially if they were 'semi pro.'

Just a suggestion tha knows.