Also I suppose some within the revival hoped that by singing 'folk songs' of a political nature they would advance the cause of the left, so they did it for nothing. And maybe felt that stuff not done for money was in some sense symbolic of anti-establishment beliefs?

On the other hand, it does seem clear that selling ballads for money was a strategy of trade unions and organised labour going back at least to the early 20th century. I have a book by Roy Palmer with some examples.

Complicated topic.