Why email a newsletter but not post it?

I have to confess publicly, at the risk of being thought rude, that I was dumbfounded to read Polly Low's comment this morning:

The absence of the latest newsletter from the BES website is deliberate — only [listserv] subscribers get the cutting-edge news!
Why on earth would a professional academic organization with a web presence and a mission statement thereon that contains the following words limit themselves in this way?:
to promote the study of inscriptions, texts and historical documents ... disseminating news of the latest developments in epigraphic studies, in Britain and around the world
Or is the real issue that "subscribers" = "members" and timely access to the newsletter is seen by the Society as an exclusive benefit of membership?