They accept cash, they just don't allow the phone to be purchased with cash alone. You could pay the $399 in cash and the tax in credit as long as they have a credit card to put into their system.
I realize that, but if I use a gift card I can give them a fake name (as I could with cash)-- with a credit card, they get your info automatically from the mag strip-- so it just doesn't make sense why you cannot pay with cash😀