I know some people prefer to keep to: recipients in reply message as they were in the original message, but I prefer the way Apple’s Mail does.
If you don’t satisfy with Apple’s, you may try to find alternatives in App Store. There are many of them, some are even free.
Some of them may meet your demands.
I was hoping you could provide a link to an apple doc that shows this as the expected behavior?
For your reference, here is the link to the internet standard, RFC 5322 (sorry, not apple’s )
http://tools.ietf.org/html/rfc5322#section-3.6.3
When a message is a reply to another message, the mailboxes of the
authors of the original message (the mailboxes in the "From:" field)
or mailboxes specified in the "Reply-To:" field (if it exists) MAY
appear in the "To:" field of the reply since these would normally be
the primary recipients of the reply. If a reply is sent to a message
that has destination fields, it is often desirable to send a copy of
the reply to all of the recipients of the message, in addition to the
author. When such a reply is formed, addresses in the "To:" and
"Cc:" fields of the original message MAY appear in the "Cc:" field of
the reply, since these are normally secondary recipients of the
reply. If a "Bcc:" field is present in the original message,
addresses in that field MAY appear in the "Bcc:" field of the reply,
but they SHOULD NOT appear in the "To:" or "Cc:" fields.
Note: Some mail applications have automatic reply commands that
include the destination addresses of the original message in the
destination addresses of the reply. How those reply commands
behave is implementation dependent and is beyond the scope of this
document. In particular, whether or not to include the original
destination addresses when the original message had a "Reply-To:"
field is not addressed here.