> If I owe you bitcoins I can choose to pay you in USD and you have to accept it.

That's not true at all. Legal tender laws mean that an offer of US dollars is legally recognized as an offer to fulfill a debt obligation; i.e. if the issue ended up in litigation, the court would treat your offer of dollars as a settlement of the debt.

But that doesn't mean that anyone is actually obligated to accept your offer of dollars, or that the agreement that generated the debt wouldn't give you a strong incentive to pay in BTC as opposed to USD.