they have made a great deal of money with this trade over the years. they are powerful enough to maneuver the market up and down in the short run, and make trading profits. eventually they are stuck with a short position which is bound to blow up in the end, but it will eventually be settled in cash, not bullion.

in addition, the bullion market is tiny compared to the other markets, i.e bonds, currencies. banks such as goldman sachs and jp morgan have made enormous profits in those other markets, and the potential losses in gold and silver are trivial in comparison. if they have to spend some money to control the price of gold and silver, that's just a cost of doing business.