Companies frequently get a background check on a potential employee and it has nothing to do with immigration status. If employers were made responsible for verifying immigration status the illegal immigrant problem would be solved.
Why are they just now doing what they've been able to do for probably decades? Good question to ask yourself...could it be that Zero has figured out he doesn't need these people any more? Could it be that the Zero Administration has really just figured out what deep shit we're in with Social Security - possibly killing his re-election (that may not be assured by the 2010 Census from his ACORN buddies by 2012?)
Regardless, they, SSA have been able to do this for a long time....just haven't been interested in enforcing it....screw them and government.