Jobs Americans won't do

According this profile (note: audio file) at National Public Radio, illegal immigrants make up just under 5% of the workforce, and tend to be more concentrated lower skilled occupations. Examples of where they are concentrated include:

• 24% of farming jobs
• 17% of cleaning jobs
• 14% of construction
• 29% of roofers
• 12% of food preparation jobs

Average earnings for a Latino male:

• undocumented, in US for 5 years: $430/wk
• legal immigrant w/green card: $700/wk
• immigrant who becomes US citizen: $930/wk

These numbers, if accurate, suggest a couple of things to me. 1st, the majority of "jobs Americans won't do" are done by, well, Americans. 2nd, the gap in earnings between illegal and legal workers is not small, probably more than the 8% figure found in the WSJ. You guys sure there isn't any exploitation going on?