The NFC feature does not sit in the battery, only the antenna does.
NFC consists of:
- an integrated chip (soldered onto the logic board)
- an NFC Antenna that needs free view to the phone's back side
(antenna signals from the sticker on the back of the battery only need to pass the back cover)
If you look at the antenna you'll notice how big it is. It's basically just a coil with 4 windings, see ifixit's teardown. NFC allows for passive devices (Credit cards and such) which need to be fed energy from an active device through the antenna. Some engineer decided its best fit would be as a sticker on the battery.
There's only 2 positions where you can place it:
- Back of the battery
- Inner side of the battery cover (like with the Nexus S)
My guess is this: By placing it on the battery, the expensive and big 2-point connector (as with the Nexus S) could be merged together with the battery contacts to make it cheaper (and/or maybe more robust).