For every utterance of "reasonable" in law you can be sure over $1B of laywer fees have been (or will be) spent.
Compare Legal Tender against an ordinary Reasonableness test. Legal Tender says that I only have to accept payment of your debt in specific forms (the "Legal Tender") and I can refuse to accept other payment.
So maybe our currency is Doodads, the Legal Tender law specifies that the 10 and 50 Doodad Coins shall be Legal Tender, and you owe me 15000 Doodads. You try to pay by card, I refuse. You try to write a cheque, I refuse. You try to pay with 150 of the 100 Doodad Coins, but again I refuse. Eventually I take you to court and... I win?! You did not pay your debt in the required Legal Tender.
With Reasonableness the court might buy that it was OK to refuse to accept the card (maybe I don't have a merchant account) and maybe even the cheque too (but already by then I expect a judge to have a lot of questions about how I thought you would pay and I'd better have a really good answer) but the 100 Doodad Coins are clearly money, with Reasonableness as our standard it's obvious that I lose my case, there's no need to write a law saying "Yeah duh, the 100 Doodad Coin is money" because a reasonable person can see that.