"Legal tender for all debts public and private" is a guarantee backed by the treasury. if you owe the restaurant a debt, they are legally obligated to accept cash tender. Note that you have to actually owe them, you can't demand they accept cash tender up front, they have the right to refuse the terms of sale. if you can successfully argue their card only policy was not successfully communicated, then you have a case. I ANAL.

That is illegal in my state. I wrote a strongly worded email to a former landlord informing them of this when they tried to pull thos shit and they immediately backed down, presumably because a bunch of other people did the same thing. It is insane how often companies do just blatantly illegal shit in hopes that nobody will notice because the penalty for getting caught is basically just pay back the people who noticed they got scammed and maybe like a $50 fine that was set when $50 was a huge amount of money.