It's a thought experiment. The question is about a case in which you know for sure. It's a big world and this is likely to happen at least sometimes. If you don't want to play along with the thought experiment, no problem. But it's a useful and interesting question if you can.

And there are also cases in which you can clearly know something is illegal. Someone purchasing guns in a country where guns are not legal doesn't need a judge, jury, and court process for a payment processor to decide they don't want to be involved. It's not arbitrary. And your right to make purchases using a credit card is not a basic human right.