It's not something I particularly care about to bother getting into an argument over but the fact still stands that the dev doesn't want his theme to be free any more. He wants to be paid for his work.
Whatever way you look at it, your getting the work he's done, for free. When he's clearly stated on his site that his work now requires the user to make a donation if they want to download it.
It's no different then downloading an MP3 from a free site is it? But artists in the music industry would call that theft wouldn't they? As essentially your taking their work from them for nothing.
I hope that helps. Lol