"How can they get SS back when the card they used is fake???"
They become legal citizens under the amnesty, then prove they worked "illegally for x years" under a stolen/fraudulent SSN.
They get a new "legal" SSN and then get credit for the years worked. Once you work x number of years, you are in the system and due SS payments when you retire and SS payments if you are disabled. Also, you get SS benefits for your kids if you die.
Then why not arrest them at that time for SS fraud?