"These are not chain restaurants, but small, privately owned restaurants. How did these people get into the country, and how did they amass enough capital to open a restaurant?"
Well, not to get too paranoid or anything, but, restaurants have historically been great places to launder money or hide income from the IRS. Not saying that these people are doing this, but I know lots of places that are "cash only" businesses. Makes it easier to cook the books.
Delphi currently pays its workers about $37 an hour in wages and $40 an hour in non taxable "benefits". Every business tries to "launder" money or hide income from the IRS.