Since the tune is traditional and Robert Burns wrote the words how can the Robert Burns Club of Milwaukie claim a copyright on it?

With a bit of experience we can all put chords to a tune but it would be a wee bit cheeky to suggest that since I worked out the chords to (for example Bridge over troubled water) I had some sort of copywrite on it.

I realise that this is threadcreep but in 30 years of playing I have often wonderd about these things when it comes to traditional music.