HowardJones,The performer has a right to his arrangements,for instance my song accompaniment tutor for concertina.I use Traditional material and a couple of self composed songs,the arrangements are my creative work,I sellthe book on my websitehttp://www.dickmiles.com.
by selling the book I am happy for people to use my arrangements for amateur performance,if it was used for alive gig as long as my name is mentioned as being the arranger Iam HAPPY,of course it is gratifying to be acknowledged,if a performer records it[the exact arrangement] I would expect Trad arr R.Miles,Not trad arr J Bloggs.
So the song may be everybodys but the arrangement isnt.
I believe this was also the attitude of Cecil Sharp.
finally the artistic purpose of my tutors is to give people an idea of how it is done,and then hopefully they develop their own style.