No ex poste facto laws. If new laws come into effect, they have a date when they become effective. What you did in the past doesn't matter.
Kind of like drinking when you were 20 when the drinking age was 18 ages ago. You're not going to get busted for that when they raised the drinking age to 21 a few years down the road.
And why not create documents that are much harder to forge and then hold employers accountable? Employees are accountable to their employer if they accept counterfeit currency. No reason that employers, if legal immigrats are given an identification that is as difficult to forge as is our currency, shouldn't nor couldn't be expected to hire only legal citizens.
You are already required in many areas to verify insurance on a contractor before you hire them. Even if it is not required, you are civilly liable should they cause injury and/or property damage and they have no insurance. Checking for papers doesn't seem like that big of a deal.
Personally it is no skin off my butt if they start throwing these people out of the country. I mow my own lawn, thank you. These suburban yuppies ought to be forced to pay the extra for documented workers. Its not my problem if their dual incomes can't support their cast of servants they like to have so they can continue deluding themselves about their real wealth.