They are trying to create a coalition that is loyal to the party. They want to be able to control votes. I don't think they are doing a very good job at this point.
There is something to be said, however, with the thought that if he's not a Democrat anymore he shouldn't caucus with them. Although I don't know Lieberman's views very well.
EDIT: Turns out he has a solidly liberal voting record minus the Iraq war.
http://lieberdem.blogspot.com/2006/0...ng-record.html
So I'll agree that it seems ridiculous to not have him caucus with the Democrats. Where is he supposed to go? He certainly isn't a Republican beyond some foreign policy views.