I agree with everything you have said above, with one exception.

The founders of rec.boats had in mind a discussion group about
"boats". It isn't "rec.boats.politics", "rec.left-wing boating
whiners", "rec.right-wing boating whiners", or
"rec.boats..whiners"....it is "rec.boats.".

Which illustrates a philosophical challenge. Should the group be self censored
as to topic? If so, how could the censorship enforced?

None of us can control or regulate the group- but we can control or regulate
ourselves. If everybody only submitted posts that reflected their own idea of
what the tone and nature of the group should be, pretty soon it will assume a
shapoe reflecting the will of the masses. :-)