um. US dollars havent been backed by anything for decades. It is just an IOU from your govt.
Steem dollars trade at roughly $1 via several markets, so regardless of how either is backed, they are convertible to each other.
Have you heard that "time is money"?
There are also new thinking that "attention is money"
steemit's currencies represent the time that the content writers and curators put in and the attention that this content gets. In the information age, this is actually closer to real value than some lump of metal you have to pay to store safely.