SBD is a directly manipulated asset, much like central banks, the witnesses control the fundamentals for it by adjusting it's monetary policy (interest) which encourages either holding it, or converting it to STEEM. Right now, it is trading about 10% under par value meaning if it continues to stay this way, the witnesses will begin to raise interest rates on it. The only reason it is currently 0% was that it traded as much as 2 dollars to 1 SBD not too long ago.