It's not spirit of the content, nor about fans or (dis)respect for the author - it's all about the money. If the original idea was popular enough someone will want to make a buck. That's motivation number one for sequels.
The author himself/herself usually doesn't have a vote in this, so there's just guessing. If an author allowed sequels during his/her lifetime, there's no reason not to continue them after his/her death. Otherwise let the stories and the ideas rest in peace.