So if you are buying Bitcoin on the futures market, you are buying a piece of paper that says you own a Bitcoin at the price the Bitcoin trades at. Unlike Coinbase where when you buy Bitcoin you can send it to another wallet or exchange. With the Bitcoin on the futures market the 'actual' Bitcoin does not exist so cannot be moved about. The trouble starts when they print more futures that actually exists so in essence they can sell 10 billion Bitcoins which can dilute the price because there is too much supply.

How this will affect the price we pay from Coinbase is yet to been known but you just have to see how the metals are trading to understand how much power the markets have to suppress the price down.