The article writer seems confused about what a "debt" is.

An entry fee is not a debt. If they let you in and sent you a bill later, then yes, they'd have to accept cash. But before they let you in, you don't owe them squat.

Maybe it's not a good idea to go digital-only. Maybe it's weird for a government agency not to accept cash. But this is an op-ed by a guy representing an industry group, and his failure to grasp the basic terminology he's using does not make me terribly compelled by his argument.