I wouldn't say only because the government enforces them.  Contracts mostly work because people mostly intend to live up to their agreements.  Invoking government intervention to force the other party to live up to the agreement is a last resort.

It's correct that monetary judgments in the United States are going to be in dollars.  However, the court might analyze the actual agreement to find out what that dollar value is.  The dollar value might not be the exact equivalent of the exchange rate between BTC and USD at the time of the agreement.  If the court has to analyze it, it's a very fact-sensitive analysis.

There's another option a court has in a contract case, which is to order one party to do whatever was contracted.  This is called "specific performance."  For instance, you're Liberace and you agreed to do a show at Carnegie Hall.  Or you're BFL, and you agreed to deliver a bunch of shit you have no intention of delivering.  Or you're Josh Zerlan, and you have agreed to suck a hundred cocks in public and swallow every time.  Courts almost never do this.  

They assign a dollar value to whatever you contracted to receive, and that is what you get in the judgment.