USD is what the creators decided to price things in on the platform. That can be changed at any time with a hard fork. It could be priced in gold, silver, pesos, rubles, yuan, or many other things. There's a Russian form of Steem called Golos which decided to price things in terms of gold.
If you were to read the original whitepaper, you'd see that SBD was created as a smart contract that was always supposed to be worth about $1. Clearly, that's not the case, but if it had worked as planned, there wouldn't be much difference between pricing it in SBD and USD.
Ok from above you said if a post earned $10 the 5 USD would be paid in SP and 5 SBD. Why not 5 SBD worth of SP?
USD is what the creators decided to price things in on the platform. That can be changed at any time with a hard fork. It could be priced in gold, silver, pesos, rubles, yuan, or many other things. There's a Russian form of Steem called Golos which decided to price things in terms of gold.
If you were to read the original whitepaper, you'd see that SBD was created as a smart contract that was always supposed to be worth about $1. Clearly, that's not the case, but if it had worked as planned, there wouldn't be much difference between pricing it in SBD and USD.