If you profit on a stock sale you are forced to pay US dollars as tax.
If you profit on a gold bar sale you are forced to pay US dollars as tax.

If you generate any sort of economic activity involving any sort of tangible or intangible goods (such as Tibanne Ltd that exchanges bitcoins), you are forced to pay US dollars as tax (or in the case of Mt. Gox, Japanese yens).

Failure to pay those taxes gets you dragged out of your house at gunpoint if necessary, and imprisoned.
How is that not forcing people to transact in, and own fiat currency?