Liberty Dollar raid

Will somebody better versed in the law tell me what's illegal about this operation? When you get past the political talk, what it looks like to me is that they issued what are essentially promissory notes backed by their own supply of silver, and those can be used as a means of exchange with any other entity willing to accept them. They're not counterfeiting – there's no pretence that the stuff is issued by the Federal Reserve – and nobody is under any obligation to treat them as currency. If it's all voluntary, private and above-board, what business is it of the FBI? I don't understand.