Surely no one is really surprised by this:

Federal officials are sweeping Wal-Mart stores across the United States as part of an effort to arrest some 300 illegal workers by Thursday evening…..
Officials say investigators are concerned about a pattern of Wal-Mart using contractors who employ illegal aliens and that Wal-Mart has, in fact, continued to use contractors who have been convicted of hiring aliens in the past.

Or that Wal-Mart would try to hide behind the ‘contractor’s employees’ argument:

“They arrested a number of members of the floor cleaning crews.” Williams emphasized the workers are employed by contractors, saying, “they’re not Wal-Mart associates.”

If Wal-Mart really did not know then there are some management types who need to be fired for failure to do due diligence in hiring the contractors.