Ask HN: Why would a foreign domain registry want my SSN?

I have a vanity domain under .pt. I bought it three years ago and paid for 5 years. Last week I received an email from the registry saying that my contact information is invalid. I looked - all was fine. I sent in a request for more info, and they told me that they need my taxpayer ID (US Social Security Number). I'm a civilian, not a Portugese or EU citizen, and the registry doesn't have anything to do with US tax authorities. I refused to provide it.

They argue that they need it to either a) validate that I'm myself if I lose control of the domain or b) because they issue invoices/receipts for purchase. Neither of these things require my SSN. I've had numerous exchanges with them, and all of them use logic to the effect of:

We don't have this information. Because we don't have it, we have decided that we need it. Because we need it, you have to provide it

My argument is:

You didn't need it when I registered. You didn't ask for it for the last 3 years. You don't need it for your two stated reasons. A US SSN is considered confidential information. Without a law stating I have to provide it, I'm not going to provide it.

The threat is that if I don't comply in 8 days from the original email, they'll delete my domain.

Why would they require my SSN? Am I right in not giving it to them? Has anyone else with a domain under .pt run into this?

Edited: for formatting