This feature does not appear to be connected to threading. When one replies, the message to which one is replying is copied as text into the bottom of the posted reply. It's a complete copy in the new message, not a reference to the original.
Messages are already threaded, so if you want to see the content of the previous/original message, all you have to do is scroll up.
And if you want to cite a specific passage from a message, you can do this using a block quote:
Michael, I'll have to look...
The automatic embedding of the entire parent thread in each reply lacks any plausible utility. It litters threads with extraneous content, and results in unnecessary manifold duplication of data.