Because you can make it so that the required certificate/signature has to meet certain criteria to work. For instance, imagine there was a PayPal equivalent type app for paying QR codes, and they required all codes to be signed by one of their business customers (who they have on file). Or with a certificate they themselves issue their customers.
Very true. You can find many cases of that though. Just the other day I was trying to get crypto quotes and accounting inside Gnucash, which has been supported by the backend API’s since forever ago, but the interface essentially doesn’t allow for it because the developers don’t consider crypto as currency, and don’t want to support custom currencies or even just using the existing data source API for anything but stocks, derivatives and fiat currencies.