We all unwittingly break many laws day. Living in the US is inherently illegal, even if you happen to be a citizen. My point is that there isn't some binary division between people that break the law and those that don't.
If businesses need to be immigration police, they'll just hire illegal immigrants under the table (moreso), and be more exploitive about it.
If people are afraid to talk to the police, they won't. This breeds gangs & protection rackets, which presumably you don't want, because they increase crime.
As these problems will become worse and worse, the more we will try to crack down on illegal immigration, intensifying said problems. The tragic irony is that the solution is the cause of the problem and it is part of a nasty feedback loop.