The government has legal privileges for some monies and legal disabilities for others. Legal tender laws are one type legal privilege. Here is a statement by the Treasury on legal tender:
http://www.treasury.gov/resource-center/faqs/currency/pages/legal-tender.aspx
On the legal disability side, U.S. courts will not enforce gold contracts:
http://www.treasurydirect.gov/instit/statreg/fraud/fraud_adams.txt
Without such legal support, fiat money would not be chosen by people over commodity money.
