Sometimes songs that have known authors (Example: When You and I Were Young Maggie) are marked as traditional because they are in public domain due to the passage of time. Sometimes artists then have the gall to attach their own copyright for things like arrangement without any acknowledgement of the true writer of the work. Sometimes true traditional material also shows a copyright mark.
My memory is a bit foggy but there was a high profile folk group doing this years ago and another high profile folk singer really took them to task for doing this. I do not wish to mention names but I'm sure that some of you remember this.
   I guess my ethical Question is: If the performer stole the material in the first place, is it permissable to in turn steal the material from them?
                Sandy