You are allowed to accept payments in different currencies, or even barter, but you can't require them.
It's only when the form of payment isn't specified, that dollars must accepted, by default.
If our contract was specifically for 1 BTC, it's legal to demand 1 BTC, and to otherwise sue for breach of contract.
After all, how else would a contract for currency exchange be enforced?
If you owed me 1 btc because I paid you 1.02 btc in dollars (2% transaction fee), it would make no sense to allow you to simply repay me with the equivalent of 1 btc in dollars, and consider that to be a settled debt.
And no - money has nothing to do with taxes and initially it was based on social agreement. Pacific stone coins weight tonns - some of the ones that are still in use physically do not exist anymore or are inaccessible in the bottom of ocean.
So from that viewpoint these cryptocurrencies all are like early monies - based on agreements and trust.
And ffs - people are weird these days - money is worth only when dealing with other people - taxes(earliest form of robbery) is not and wont be one of the reasons for money to exist. Goverment, that takes care for people and decide what they have to do and think don't need money to run society. Money is needed when independent people/souzerains deal with other people - slaves and children and sick/unsane dependant people and pet animals do not need money as they have no voice in these matters