authenicating blog comments in academia

More of an observation this one:

I recently tweeted a link from the University of Michigan Press's Typepad blog, and quite properly if you want to post the comment they ask you to authenticate, but how they ask you to authenticate is interesting:

1. Twitter
2. Facebook
3. OpenId
4. Wordpress
5. etc

but no mention of using a UMich id to authenticate. Now allowing non UMich people to authenticate to comment is a good idea, but I would have though the'd have shoe horned in Shibboleth or something like that to allow people at other institutions to comment using their institutional credentials.

But no, nothing like that. So the question is accident or design - do they reckon facebook and twitter so pervasive they unconsciously expect everyone to have an account?