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You might want to take a look at some of the research The Pew Hispanic Research Center has done about the concentration of undocumented workers in several specific occupations and industries.
Roofing in particular.
The percentage of illegal workers in some occupations (such as roofing) runs as much as 4X the percentage of illegal immigrants in the overall population.
So yes, it might very well be worth the effort of cracking down on "Joe Smoe's Roofing Company".
As far as Conagra?
LARGE FOOD COMPANY RECEIVES MASSIVE FINE FOR EMPLOYING ILLEGAL WORKERS
"ConAgra Inc., one of the nation's largest food manufacturing companies, agreed to pay a fine of $225,000 for knowingly hiring illegal workers at the company's chicken plant in Kentucky...
...The announcement of the fine comes on the heels of a report issued by Congress which estimated that as many as 25 percent of the workers at Midwestern meatpacking plants are working illegally.
So, yes, huge corporations can be prosecuted for hiring illegal workers.