This is more to do with Yahoo Groups than Google Groups but they seem similar. Recently I've joined several Yahoo Groups about specialized ham radio topics. Nearly all of them keep their archives private. I have apply to join (basically push a button and say who I am) and then wait for approval from the admin. Once approved I can read the archives and also post. Posting from members is usually unmoderated. It's painless enough but still very frustrating when I'm just searching around for information and a quick look at the archives is probably all I want.
I don't mind having to join if I want to post but do they achieve anything by keeping the archives private? Yahoo obscure the email addresses so spammers' 'bots are not going to get much from them. I've asked several admins "why do you keep the archives private?" and have not received a convincing answer. It usually goes something like "I understand your frustration but we have a lot of trouble with spam" and sometimes goes on to imply what a silly question I asked. Well ... I still don't see how keeping the archives private helps to reduce spam. I haven't been a group admin so maybe I'm missing something.
I can understand keeping archives private or non-existent for a group on a personal or private subject but that doesn't apply to these groups.
My guess is that this is Yahoo's default setting when a group is created and few admins really think about it. Of course Yahoo want as many people as possible to join.