each bank uses a single cylinder head, therefore the pistons have to have a 15 degree bevel to keep the deck surface flat. otherwise you'd be dealing with a head that has a 15 degree angle down the middle, which would lie directly in some of the cylinders, so it would have to alternate back and forth, and be very expensive and hard to get them to seal... think about it...
if they were just flat you could get very little compression, as the squish would be normal on one side and huge on the other, it wouldn't work at all.