Yes, Pamela R., you have touched on a sore point. Folkies can tell the venue that they will only do PD songs, but somebody will come along and break that promise, I guarantee.

I used to be an officer in a small non-profit that brought big bands to town, most from Scotland and Ireland. No way could we afford ASCAP and BMI licenses. We put it in every contract that music should be PD, and the bands consistently and blythely ignored it.

There will always be somebody who doesn't know about licensing. Or has just had a few beers, is feeling defiant, and performs a copyright work anyway.
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I was also an officer in a dulcimer club which wanted to publish a book of hymns. Despite my insistence that the hymns be PD, members of fundamentalist groups tried to slip their personal, copyrighted favorites past me. (They were not officers, so any trouble would not affect them.)