SSN and NIN could reasonably be used for identification of the form of "this person's data is different from that person's data". (identification as in primary/unique key)
They can't/shouldn't be used for authentication that this person is who they claim to be. (identification as in password)
100% agreed, perhaps I should have worded my comment better. My point was that this seems to be common practice in the US but not over here.
SSN's were only ever meant to be used for identification (as in a username). In the beginning the worst that could happen is that someone else would file their payroll taxes with your SSN, and make you eligible for more Social Security benefits when you retired. It was never meant to be a password.
For employment, the I-9 form is what we fill out as authorized US workers and to document same.
https://www.uscis.gov/i-9 (the "paper version" link works best)
You'll note that Social Security card is in List C "documents that establish employment authorization" and not in List B "documents that establish identity" nor in List A "documents that establish both identity and employment authorization"
I guess you can look at it as a form of off-line multi-factor authentication. You are using the Social Security card as proof of eligibility for the person whose identity you have already established using another factor. In that case, it's the card that acts as the factor, not the number (which if you've ever seen a social security card, it's ridiculous to think that it's in any way secure or even durable).