Because only Congress has the power to mint coin? Well, according to that law, if the intent was that the coin be used for currency you cannot mint it. It is a really dumb law. So as long as the intent was for the coins to be used as currency: no good. Well, I'm sure the IRS does in fact hate bartering because of how much more difficult it is to track the transactions, however, they are all taxable transactions. If you want me to dig further I can, but I can't remember the specifics except that there was a court case involving a railroad bridge.