As has been pointed out, the non-member messages can be moderated.
yes they can. but this requires a moderator who has the time to do
it, who can consistently do it in a timely manner, who acts as a
spam filter rather censoring content with which he/she does not
agree, and who is trusted by everyone (or very nearly everyone)
who wants to participate on the list.
and yes this is much better than insisting that people subscribe
to a list in order to post. and it has worked fairly well for
a number of the lists that I run.
but I think it would be difficult to find a moderator for the
ietf list that meets the above criteria.
Keith