When they allow people to come across the border, the only way they can get a real job they must provide a Social Security Number. Now once they have that number they are in the system and the government uses that the same way as they do others. Once you are registered, they use your identity and place you on the market and use your ability to earn money and also use you (your identity) as collateral on the national debt.
It works the same way with automobiles, and in large cities like Chicago they even require you to register your bicycle under the pretense that if it is ever stolen and recovered it can be returned to its proper owner. But I know of no one who ever had their bike stolen that ever got it back.
The hicks in this small town I live in still have not figured out that one with bicycles as yet. ;)