"Why would he be considered more than an editor at best?"
Take the works of Shakespeare. Each play is, in its original version, copyright free; but you, as an editor, provide the modern reader with updated punctuation, consistent spelling, explanations of archaic words and expressions, historical notes on the characters and settings - and so on. What is copyrighted here is the contribution the editor has made - because he or she has created a new edition of the play.
Personally I think that the Anne Frank copyright renewal finagle stinks to high heaven - but the underlying principle is well-established, at least in English law.