Genuinely, what ever possessed the people at Tumblr that convinced them that being able to edit other people’s posts was a good idea?

You were never able to "edit other people's posts", per se – it's actually much dumber. Tumblr initially had no ability associate posts with other posts whatsoever, so when they added the ability to "reblog", they implemented it in the simplest possible way: by copying the text of the source post into a new post by the reblogging user. You weren't really displaying other people's posts on your…