The idea is that the peg is a peg because it is pegged to Steem (not to the USD). The guarantee is that you will (almost) always get $1 worth of Steem for 1SBD.
Important about that is that SBD is pegged to Steem, not to anything else. As such, endless that peg is enforced in the code (and possible also actually tethered to an equivalent amount in real world value) the free market will decide the value of the SBD. In other words... traders.
Check out this now two years article by @dan to get an insight in what the vision was.