Actually, when I sit in a restaurant, as I did, and have just finished my meal, as I have, and ask to pay via card (as the sticker in the lobby told me is possible), I have incurred a debt, which I am willing to pay, either by card, or should that not be possible, with cash as a legal tender fallback.

In pubs that are card-only, pay-first, you might have an argument - but this may become a discrimination lawsuit the moment my card gets denied when the sticker says it should be accepted.