But the speculator sells the future before settlement day. If all the speculators were bidding up the price, but not taking delivery of the oil, refineries could wait until the speculators rushed to sell their contracts and buy them up cheap.
Refineries can’t play games like that. If they did, and it turned out badly, their customers (you and me) would have no gasoline to fill their gas tanks.
So it really doesn’t matter if the speculator sells at a later date, as those (such as refineries) have already secured their supply of oil... at whatever price was high enough to outbid the speculators.