In a site that uses the "Announcement" tool but does not use the "e-mail archive" tool, announcements still show the archive address as the "to: " line in the e-mail that is sent out. This should be on the "bcc:" line. Participants who use "reply all" will receive a "bounce" error message, this address is not meant to used publicly in these cases and should be hidden.
In sites that do use the "e-mail archive", reply-all to announcements will send the message to the archive tool and to all participants in the site, this is not necessarily the expected behavior since announcements are conceptually distinct from the mailing list. It aggravates the perennial listserv problem of accidentally replying to the list.