From what I understand... Both the founders, ned & dan, have virtually all of their STEEM vested as Steem Power. It takes 2 years (104 weekly payments) to convert Steam Power into STEEM, the tokens which can be sold on an exchange. So they could only cash out 1/104 of their STEEM per week. And since it's a transparent public blockchain, everyone would notice if they started cashing out. Kind of like if Satoshi Nakamoto were to spend his bitcoins.