That's not entirely true. It's perfectly legal to specify contractual terms of payment, including what form it must take.
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.
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.