Well, since capitalism is perfect, rather than address the question "Why does capitalism do this?", let's instead address the question "Why is this the sensible thing for a society to do?" — which, we understand, is the same question.
Suppose you're a society, and you have what you regard as a dependable source of 10 barrels of oil a day, which you dump into the Big Box of Refinement so that, three months later, out comes gasoline. Suddenly, you receive information suggesting that you can only count on 5 barrels a day for the foreseeable future; do you keep consuming 10 barrels' worth of gasoline a day, waiting three months to conserve, or do you immediately cut your use to, say, six barrels' worth, suspending the least valuable uses, so that you can continue to use more than five for a while into the future? The latter course of action makes far more sense, and the immediate rationing is what we see; prices go up, discouraging the least valuable uses of gasoline, conserving it for future uses.
If the source suddenly proves capable of delivering 20 barrels of oil a day, it would similarly make sense to increase your usage immediately, provided that's physically possible. If the black box is still only churning out 10 barrels' worth a day for now, it isn't physically possible. If you have a small stockpile, you can draw on the amount of the stockpile until the new oil works its way through the system, but you are limited by the size of the stockpile, and you probably don't want to run it completely dry even at that.
The asymmetry, then, stems from the fact that it is easier to export oil and gasoline to the future than to import them from there to the present. As soon as anticipated future prices exceed current prices by more than the various costs of storing the commodity, entities will start buying current supplies for the sole purpose of sitting on them until they can resell later at a profit; this will drive up the current price and drive down the future price, much as the import of cheaper goods from Japan brings prices toward a geographic equality. There is no similar mechanism for limitting how low future prices can be compared to current prices, except to the extent that entities have readily available stockpiles — such as the National Strategic Petroleum Reserve — and are willing to sell them now and buy them back later at a profit. If the stockpiles are there, they're there for a reason, and the willingness to let them go will not be complete and immediate.
If you believe capitalism might occasionally have some small imperfections, a more direct answer to the original question should now be clear: the reason prices go up immediately is because I have no interest in selling gasoline cheaply now if I can sit on it into the future and sell it more expensively then; on the other hand, if it will be getting cheaper in the future and I want or need it now, that does me little good.
It should be clear, too, that this has nothing to do with monopolies or cartels. If you're selling oil for $20 a barrel, and the price for delivery in two months is $40, I'm going to raise my price; if you undersell me, keeping your price at $20, I have no real problem with that. I'm more than willing to let you run out your supplies while I sit on mine and take the extra $20 two months from now. Perhaps you can get some favorable publicity for selling at the old price; this can be analyzed pretty much the same way as any other sale to build name recognition and brand. The fact is that the market price has moved, and there's no good reason for performing an economic analysis clinging to the price that the new information has rendered obsolete.