What's going on technically is they've built a list of IP addresses of their members, and are blocking access from other IPs.
What's going on socially is kind of weird, keeping the riff-raff out, but in a way that means they will not acquire any new members, unless they've set up an alternative screening mechanism.
Not every site on the World Wide Web is open to the public (but what's the point of being on the WWW?).
Not every Internet domain is a site on the WWW. If other protocols are in use, a web site may be set up to reject all WWW probes.