To send to a mailing list, the sender must either have a copy of the list or
the system managing the list must decrypt and re-encrypt the
message. Neither of these is a good fit with the current email architecture.
The former is secure but unwieldy; the latter is reasonably
efficient but breaks the desired end-to-end security.
FYI, the Sympa list manager which is widely used in Europe does the
latter, S/MIME key for the list, and the list software re-encrypts the
messages to the recipients' keys.
Given a choice between trusting the list software and trusting all of
the subscribers, that seems a reasonable way to do it.
R's,
John