An over simplification. Usually batteries are quite large and easy to sit alongside one edge of a player. All that is needed is a thin divider, a contact, and obviously a removable portion of the case (albeit a simplification on my part). It should be relatively easy to design this in on most players without compomising the design or the case size by a very few millimetres. Without seeing the "guts" of the Karma it is difficult to say, but having seen some other players this has been the case. Eitherway, you have your view and I have mine, and I suspect neither view will change :-)

All-in-all though this is a somewhat moot point without knowing the cost of replacing the battery by Rio, the likely availability of the battery for years to come, and whether the battery can be sourced elsewhere and replaced by a savvy user.