US dollar is measure of good faith in the US, country as a whole, backed by her credibility, and you know it US is very well-endowed country (relative speaking). I think it's not "In God we trust". rather it's "In dollars we trust", so everyone can feel safe to exchange goods and services around the world. If bitcoins are backed up gazillionaires of the world, then it might have credibility or else they are just as good as Zimbabwe dollars or perception.
Article 1 - The Legislative Branch
Section 8 - Powers of Congress
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
To borrow money on the credit of the United States;
To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;
To establish an uniform Rule of Naturalization, and uniform Laws on the subject of Bankruptcies throughout the United States;
To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;
To provide for the Punishment of counterfeiting the Securities and current Coin of the United States ;
To establish Post Offices and Post Roads;
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective."
Section 10: Limits on the States[edit]
Clause 1: Contracts Clause[edit]
Main article: Contract Clause
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
States may not exercise certain powers reserved for the federal government: they may not enter into treaties, alliances or confederations, grant letters of marque or reprisal, coin money or issue bills of credit (such as currency). Furthermore, no state may make anything but gold and silver coin a tender in payment of debts, which expressly forbids any state government (but not the federal government[62]) from "making a tender" (i.e., authorizing something that may be offered in payment[63]) of any type or form of money to meet any financial obligation,[64] unless that form of money is coins made of gold or silver (or a medium of exchange backed by and redeemable in gold or silver coins, as noted in Farmers & Merchants Bank v. Federal Reserve Bank[65]). Much of this clause is devoted to preventing the States from using or creating any currency other than that created by Congress.
