This is really one of the main things that distinguishes folk as an idiom from other musical genres. The small (or large) changes introduced by individual singers or musicians are assessed, and retained or politely forgotten, by other singers/ musicians. It's a sort of peer review, and each accepted performance is the starting point for further creation.

When somebody has copyright, they can legally enforce it, but it's then taken out of the folk process, and is (merely) a performance song. A wise composer would get of their high horse and let it develop.