I believe, and this is just my view and not necessarily anything to do with why the batteries look like they do, that if the battery is non-user-replaceable, then a whole different set of regulations covering the battery housing apply.

eg: a user removable battery has to be able to pass drop tests as a bare pack. One that is built in does not need to conform to this requirement, which means it can be made with less casing/mechanical support and hence higher capacity.

Hugo

(edit: ok, so having read the thread through now, I see others have already put the above forward as an explanation. Battery regulations are necessarily frightening things, given how dangerous a badly made pack can be)