“Derivative work” is a term of art and does not just mean “inspired by” or “influenced by”.
They stole first, and then got protected by law.
The original snow white story as written down by the Brothers Grimm is not under copyright and Disney can't do anything to stop you using it.
But lot's of what is associated with that is actually original stuff by the Mouse and therefore (still) copyrighted or trademarked.
In particular the well known blue-yellow dress, red bow in her hair, the dwarves appaerences and names are all not part of the traditional fairy tale.
Also in that story snow white is a child roughly 7 years old.