The bikeshedding here came from a fundamental disagreement about usability. It really could not be helped. Some wanted the email client to automatically detect a situation where encrypted email was possible. Others felt that the risk of a user unknowingly sending a CC unencrypted was too high.

Part of this ended up coming from the fact that Thunderbird was not clearly showing the user that the email to a particular recipient was or was not going to be sent encrypted. I am not clear from skimming the discussion that this fundamental issue has yet been fixed.