Of course, if they took the time to add this recipient to their address book, they could reduce the amount of typing to a minimum. I'll take munged Reply-To headers every time, thanks. Some administrators claim that munging Reply-To headers is harmful because it surprises people, and can cause damage when things go awry. They assert that administrators should prevent the possibility of a private message being mistakenly broadcast to the entire list. This is simply not the responsibility of the administrator.
People are responsible for their own mistakes.
If someone is sending a private email which is derogatory, or otherwise embarrassing were it to be made public, they should probably be sending it directly, rather than as a reply to a public message. They should also pause and think about whether they should be sending it at all. This pause should be quite sufficient for a conscientious person using a reasonable mailer to catch any mistake that they might be about to make. In any case, it is an entirely trivial matter for the list administrator to provide an obvious clue in the subject line of every message that the message was received from a mailing list. Most mailing lists are intended to facilitate discussion on a given topic.