When my wife worked for a restaurant, most of the back of the restaurant staff were illegal and unlike what some of the people here are saying, they all paid taxes and since they were using sketchy means (SS# that weren't theirs) they couldn't file tax returns to get any of it back.
Also, look at Alabama. After they passed their tough immigration laws there isn't anyone willing to pick the tomatoes so apparently the tomatoes are rotting in the fields... From what I've heard, the farmers have raised their wages to above minimum wage and still can't attract legal workers. I can't blame them, I've picked grapes for a few weeks in New Zealand (for better than minimum wage) and it's hard, boring, unrewarding work. I can't even imagine how badly picking something like potatoes, lettuce or strawberries would destroy your back...