Thanks for your question. The short answer is that Steemit does not back the value of Steem Dollars.

To illustrate that Steemit and Steem are separate, it's worth seeing that Steemit is just one application plugged into Steem. Another application showing the same data is steemd.com.

Steem Dollars are a blockchain based contract in which the Steem blockchain has committed to buy and destroy the contract in exchange for Steem at the value of a dollar. The system does not depend on a third party to 'back' the value. The value comes from speculation (see: Steem on Bittrex.com or its market cap on coinmarketcap.com), price feeds and recurring seignorage to reward holders of Steem Dollars.