There is a certain simplicity to it, at least at the core. As I understand it, if you are not a citizen of the U.S., there is a clear path to either gaining citizenship or gaining permission from the government for living and/or working here. If you are a resident, and you haven't followed one of these paths, you are breaking the law.

Now, it does it get kinda murky when we dig a little deeper. Rod Dreher points out (as many others have) that much of our economy is predicated on cheap, often illegal labor. Food, housing, and many services are provided by illegal immigrants. Additionally, many of these workers (or their families) are taking advantage of social services that are funding by taxpayers (now, it can be said that since we're taking advantage of the workers themselves, this equation perhaps balances out). Passing this immigration bill (or some flavor of it) only attempts to solve part of the problem. Has anyone thought past this to the effects that will be felt in the economy when a pool of cheap labor suddenly shrinks?

While not a crime, it was a big problem when companies had been storing all their profits overseas. So we declared a holiday on tax so they'd bring their revenue back to the states where it could go legit and flow back into the economy.

Kinda the same idea: let's take something underground and legitimize it. Similarly, there's the end of prohibition. (and we can extend that to arguments in favor of drug legalization or prostitution)

Or, finally, there were the crime and reconciliation councils in South Africa and the Balkans. People got a pass if they just came forward and admitted their wrong doing. A similar logic is at work in the occasional tax-cheat holidays that have been declared in some states: come clean, pay your tab, and you'll be immune from criminal sanction.