No one has to buy the bitcoins.

Think about it like that, the CME group that enables bitcoin futures, actually brings you the option to bet whether bitcoin will go high or low from the current price.
As it enables high leverage, it helps you to trade at high volume even if you own a small collateral.

You bet against other peoples, you can choose to sell/buy the future contract, and in order to sell the contract you need to have another person to buy it from you, and also the opposite.