But you don't -- and can't -- know that the payment is illegal. You merely suspect it, and since you are not a court of law, a judge, or a jury, you should have no business arbitrarily cutting off someone's ability to transact.

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.

That was my comment. I intended it to mean the government shouldn’t be deputizing payment processors to do its dirty work. There were some good points in the responses and a few ridiculous scenarios. Even the ridiculous scenarios point out real issues.

In reality I don’t think there is any chance of a law requiring payment processors to process all legal transactions and definitely nothing about illegal transactions. If that were a possibility, I would back off my statement that they should be forced to process illegal transactions.