The backend of Steemit is the Steem blockchain, which is updated by a group of witnesses voted into power. These witnesses set the rules of the Steem blockchain.

For example, currently when you are rewarded for posts SBD is treated as if it has a $1 value. This means if the network wants to pay you $10 worth of SBD, it gives you 10 SBD. The actual value of SBD right now is USD$8.55, so the witnesses could update the network to reward SBD valued as such, instead of $1. This means if the network tries to pay you $10 worth of SBD, you would only get a little over 1 SBD.