well maybe you WANT to sing the song.
That's the name of the game - we go to folk clubs because we WANT to sing folk songs. Not to make vast fortunes, but because we like individual folksongs.

so why would any reasonable singer believe that no one else was allowed to sing a song that they were singing regularly.

if they want to sing it first - they should grab an earlier spot'
i think its really cheeky saying that you have got exclusive rights to singing a song - even your own song that you've written.