Just so there's no misunderstanding, the United States does not legally allow, accept nor condone the employment of illegal immigrants within the United States. Of course, even with that in mind that doesn't keep certain companies and places within our borders from doing it anyway. Our problem is that we have more people coming into this country illegally than we do legally (asylees, refugees) and we can't keep up with it, or figure out a way to fix the issue and prevent it from happening. Border control has been a hot campaign issue for the last decade, but nothing has came up to fix it.
Nor do we. Jobs here usually require proof of ID like a driver's licence or something to say that you've been granted permission by the government to work here for x amount of years (Visas).
Yeh sorry, I was supposed to imply what you said about specific companies employment of illegal immigrants.