The list is moderated for a couple reasons.
First, it's early, so I like to know everyone who's joining. By limiting access to folks who specify their interest, I know that everyone here gets that this group is about a spec -- and not just talking about lifestreaming apps or software -- and can help set their expectations.
Occasionally folks join who have specific contributions to make and I like to personally welcome them.
Initially I kept the group totally closed, focused only on implementors (i.e. folks who had sites that published activities) but over time opened up the list for anyone to read.
In the lists that I've started and maintain (OAuth among them) I find that this initial hurdle not only keeps out spammers, but also increases the quality of participation and helps to introduce people to the list.
We recently switched OAuth moderation off and moderate first posters, but I find this much less satisfying as I have less visibility into who is joining and what their interest is.
I intend to continue moderating group membership until we hit around 500, then we can consider moving to open membership.
Chris