you want your witnesses to have bias so you get more of your payout in SBD
You want your witnesses to have bias so you get an equal amount of SBD and STEEM, and so that the value of SBD is less than that of STEEM, at exactly 1 USD per SBD and a variable rate for STEEM.
Well yes, that's the real reason. But people will listen to a personal argument (my payout has more value) when an economic argument (it supports the peg) makes their eyes glaze over.