That’s the peril of being a leaf node. The same thing was true in the early days of email. An end user, or in your case something close to it, couldn’t blacklist his upstream—-as you say that would be ridiculous. But your upstream (or possibly their upstream) does have true peers that they could blacklist for bad behavior. It might mean there customers couldn’t receive calls from entire countries, but that’s a trade-off I, and I assume plenty of other, would be happy to make and even pay extra for.

This assumes that is that there isn’t some kind of international treaty that forbids doing so. The existence of such has been claimed from time to time in similar discussions but I have yet to see an actual citation.