I don’t think that posting this as a status on FB means suddenly that anyone who wants to go can, unless the status specifically says so. I think the guy was rude to presume he could just come along all because he saw it as a status.
However, what about if someone posts this in a FB event? Whenever someone creates or RSVP’s to a FB event, then it’s published to their wall. So if my friend Mary RSVP’s to an event, I see it as it has shown up on her wall, and I decide I want to go as well, can I just do that, too? Of course I think that depends on what kind of event it is. For instance, if it’s something public, like a play, I think it’s okay for me to RSVP that I’m attending because it’s a public event. If it’s a private gathering, not so sure. In one way it’s being made public because it’s an event that’s showing up everywhere, however, on the other hand, it’s something of a more personal nature, like a birthday party for someone or similar.