[previously asked on mailing list 08/19/14; but got no response]
Sent messages saved in a folder can't be re-edited as they can when left in the real "Sent" folder. Sent messages saved in another folder are identifiable as being "sent" messages, so why not treat these the same way as though they were in the Sent folder? After all, they are there because of the setting "save copy of outgoing messages to this folder instead of sent".
Just noticed this has also been reported in Debian BTS some time ago: https://bugs.debian.org/830291