In terms of the recorded song it is more about the copyright of the recording - ie to prevent copying etc of CD's. Most artists who record trad stuff are willing to accept that they have done so, but (to my knowledge at least) no one has ever tried to then prevent others from doing their own interpretations of the same trad tune/song.

In that respect the copyrighting is, I believe, quite justified. In the same way a photographer owns the 'intellectual rights' to his work rather then the person photographed, so the recording artist maintains the 'intellectual rights' over his/her work.