Also, to answer your original question, the batteries are not interchangable between models, as both the batteries and the connectors have changed between those models.
As for why these batteries are not user-serviceable, they lack a significant amount of the protective plastic sheathing that would normally be on a battery (plus the aluminum that Apple had been using around them as well) in order to increase the size of the actual battery. This has resulted in a significant increase in battery life over the older models, but comes at the cost of user-serviceability. Whether you like that tradeoff or not, it is not a BS excuse, but a legitimate safety concern that caused Apple to label the batteries not user-serviceable.
jW