Some restauranteurs and other cash businesses try and pull fast ones on the IRS. So the IRS conducts audits and has agents case suspect establishments. No reason DHS/ICE, whatever cannot do the same and jail fraudsters. If they, the Feds, can jail Martha Stewart on unrelated technicalities, they can jail these fraudsters. Start at the top, target big ones and work your way down to the small ones (they'll get the news and come correct).

Martha Stewart, before she was a star, worked as a stockbroker and knew or should have known what she did was wrong.

Could be, but they didn’t get her on anything related stock trading and instead got her for “lying”. My point is, even if they are unable to catch them red handed (you’d have to be inept), ghetto can always find alternative methods to find someone guilty.

Is your position that lying to federal investigators is okay and should not be punished?

Sure, so what we can do is punish employers who are found later to have employed people illegally.

IE, if you get "tricked" via a stolen SSN, then the business gets punished regardless.

It's my understanding that farmers avoid this by subcontracting. They don't hire migrant workers themselves. When you're desperate to get the harvest in, you don't vet your subcontractors too closely if they say they can do the job.