Hi Zero!
The caliper drop adapter mounted to the seatstay bridge is used for mounting the rear caliper; no v-brake studs are included, so a rear v-brake isn't possible; only an alternative disc.
I'm guessing -- and it really is just a guess -- the rear stays are so light, better braking can be had with a disc than V's, and it allows maximum versatility with the caliper drop adapter.
Best,
Dan.