Copying the headers make sense even if you rewrite the implementation from scratch, it gives a very basic layer of verification that your implementation matches

Copying the headers make sense even if you rewrite the implementation from scratch, it gives a very basic layer of verification that your implementation matches the original (if the declarations in the prototype clash with your rewrite, there's probably something wrong in there). It doesn't sound like a particularly bad or unethical technical decision to me, at least as long as it's done otherwise completely legally (which has been a very expensive question to settle). Copy/pasting docs and rewording everything to try and hide that fact is a lot lazier and shadier IMO.