More importantly, they are placing the battery such that it doesn't interfere with the cellular radio.It looks weird, but I can see what they were trying to do. Keep the uniform look, keep it looking symmetrical, stick a solid battery in there, while still making the top easy enough to bend to get the phone out.
"Apple says that other cases often impair cellular performance as the battery gets in the way of the radios. For the Smart Battery Case, Apple has incorporated a passive antenna which reroutes radio frequencies and minimizes interference."