Actually a priave or public business can legally use it as an ID number provided you give them permission to use it and so long as they inform you that you are not required to give them your SSN and can request a different ID be used. This is the legal loophole for the business. Once you surrender your right to privacy for your SSN, the person you give it to can use it anyway they want. Check the law and you will see that I am correct. I went through this for many months with lawyers at the hospital I worked at to make sure that SSN usage for employee IDs was in compliance with SSA law.
So the fundamental point is that an SSN can be used if you permit such use, but it cannot be mandated?