My understanding is a 1:1 peg is optional. And there is no mention of pegging the block rewards which would also be required to maintain a 1:1 pegged relationship.
1:1 is optional indeed but then the sidechain is no more a BTC application specific chain but a sidecoin, the younger brother of the altcoin.
Pegging the block reward? I think this explains your misunderstanding.
A 1:1 sidechain is not created with any unit in it. There are no block reward issued coins on a 1:1 sidechain, only coins that are locked by their owners in that chain.