Are airlines breaking the law?

Have you noticed that most airlines don’t accept cash these days for inflight things like drinks and food?

I totally get the logic of not wanting to make change for people for all of the food items.  It’s a great, simple rethink that makes those transactions so much easier for everyone.  Everyone that is except people who don’t have credit cards.

But is this really legal?

I don’t understand how it can be, given what every note of US currency has in the upper right corner, in all CAPS no less. ALL DEBTS PUBLIC AND PRIVATE is pretty unambiguous.

And we are told that federal laws apply to flights, since they tell us it’s a violation of Federal Aviation Association law to smoke on a plane.  They even say that on international flights which I find ironic (how do US federal laws apply to flights leaving Singapore, for example), but that’s a topic for another day.

I have wanted to ask this on flights before, but worry that if I ask the question, the air marshal will put me in a headlock and then the dreaded “list” that gets you special treatment at the security line.  I may now be immune to that list because I signed up for the clearme.com service which is AWESOME, by the way – no more TSA lines – now they just need to be in more airports.

So please, explain to me how this is legal.

-Ric