I still don't understand why you are mentioning legal tender in this discussion. Legal tender is cash, credit cards are not legal tender. Therefore Amazon does

I still don't understand why you are mentioning legal tender in this discussion. Legal tender is cash, credit cards are not legal tender. Therefore Amazon does not accept legal tender. Yes, businesses like Amazon could demand payment in legal tender (or credit cards, or Bitcoin) but they are under no obligation to accept any of those forms of payment. Bitcoin is no different than credit cards with respect to legal tender.