Legal tender laws are laws requiring people to honor as money things roleplaying as money while actual money is demonetized. Gold and silver had been written into the founding documents of many countries (including the United States), and that was because the people themselves had been using gold and silver as money. When gold and silver were demonetized, tokens made from various base metal compositions were imposed on the people by their respective governments. Cupronickel slugs made to look like coinage are given the status of money while 90%-silver coinage are treated as curiosities rather than as money.

It's true that legal tender currency must be honored by everyone, but the limits on cash aren't related to their status as legal tender. I'm the guy who pays for a candy bar at Target using a $100 bill because the self-checkout machine accepts it, but at the corner grocery store I use more expected bills for payment because the grocer lacks the capacity to deal with the $100 bill. At the same time, I cannot pay for a $20-dollar meal using 40 rolls of cents. The cents are legal tender, but that quantity is too much for one transaction.

In the end, money is whatever medium of exchange we agree to use when we trade one good for another. Money also predates government, so money is pre-political. Cryptocurrency should be free of the legal tebder regime.