SBD could be pegged on a downside - you always could exchange 1 SBD to 1 USD worth of STEEM.
SBD is not pegged on an upside. To do that you need to be able to print limitless amount of SBD and dump it to feed any demand. Is steemit.inc do so or a community as a whole incentivized to behave such way then it would be peg to 1 usd.
Good point, reality is indeed different . Thanks for your comment