I don't see a big deal in modifying others tilesets, as long you give credit to the original creators. The original version is always there. And no, I wouldn't care if others edited my work/edits, as long there's no plagiarism. There's a difference in modifying a physical piece of work and a digital one.

People often don't ask for permission because there is no way to get in contact with the original creators anymore. Or in Nicks case, since he made the sets for JJ2, we tend to think that it's okay to make conversions/extracts out of them. Honestly he'd probably be happy that we fixed the masking and tile arrangement.