The steem system doesn't treat steemdollars like 1 USD. See below.
Steem dollars are basically IOUs for however many steem tokens it takes to give you a USD worth of value when you cash out. If steem tokens are trading for $0.50 and you go to cash out one steem dollar the steemblockchain protocal will give you 2 steem tokens in exchange for your 1 steemdollar leaving you with 1 USD worth of steem token.
You just contradicted yourself and confirmed what I said. :]