The difference is that the "medium of exchange" is taxed.

For example: If the merchant accepts dollars and holds onto them and eventually uses them to buy new supply (i.e. sells dollars), he doesnt have to pay capital gains taxes on a possible increase in the dollars value. If the dollar trades higher versus the euro, the government does not force you to pay taxes on these exchange rate "gains".

However if the merchant accepts bitcoin and holds on them and eventually uses them to buy new supply (i.e. sells bitcoins), he will have to pay capital gains taxes.