"Nicking tunes from everywhere" - nicking implies taking something away so that it's not where it was before you took it. You can't do that with a tune. What you do is use it.

And using and maybe modifying existing tunes is what people have always done when they've made up a song, or wanted to sing a set of words they'd come across. And that's what Dylan did, the same as Woody Guthrie before him, or Ewan MacColl much of the time.

When we talk we use the same sounds as other people do, and the same words we share with them, and we put them in a different order, and we quote stuff other people have used. If we are making a collage we use bits of other people's pictures and stick them together in different contexts. It's not really all that different.

(And it's not exactly the same either - pretty well any time you make a comparison or draw an analogy you seem to have someone leap up and suggest that you've said the two things involved are exactly the same. I thought I'd try to head that off in advance this time.).