The different between IRS and any other merchant is the use of force. While you have the choice of paying any other merchant in their prefered currency, you are forced to pay IRS in their choice of currency.
You actually don't have the choice of paying any other merchant in YOUR preferred currency. You have to pay it in THEIR preferred currency, or not get the service at all. Same as with IRS.
Dollars do not have any inherent properties that make them good as money, but their government enforced "not going to jail"-properties are valueable to most people. So just like people are prepared to pay for the "get out of jail free"-card in monopoly, they are prepared to use dollars for regular transactions because they know they will face the ultimate choice of paying dollars to IRS or go to jail.
I could argue that the main difference between government and merchants is who has the bigger guns. While a merchant can't stick you in jail, if they don't like your currency, they can kick you out of a hotel or your apartment, refuse to sell you food, and refuse to sell you lifesaving medicine, which could all result in punishments similar to what a government can dole out. So, bigger guns, bigger say in what everyone needs to pay. And, similarly, if your merchant only accepts euros while you only have dollars, you can always just exchange your dollars for euros, just as you can always just exchange your bitcoin for dollars when paying the IRS. Again, really nothing special here except for the personal preference of the biggest merchant on the block.