According to 8 USC 1324, it looks like what you're talking about could quickly land you in prison for up to 5 years. So, it's a scheme maybe, but not a legal one.
The companies are doing this "unknowingly". Of course they're not stupid, they're fully aware they're hiring a lot of undocumented people. But nobody, on record, knows that. To them, all their employees have documents which appear valid. It is pretty easy to forge I-9 documents.
Part of this is because the US has incredibly poor identification infrastructure. Often, not even US citizens can be reliably identified, as seen in commercial banking. The other part is that companies cannot choose to not hire people because they think they are undocumented. You can't hire someone because they're too brown and not American sounding enough.
That's not to say that I think the solution is universal identification or legalization of racism in hiring practices.
But, what about this one?
18 U.S.C. § 1546
This says, "having reason to know." So, you actually don't have to really know, you just have to have reason to know, to be incarcerated. That really seems like I should check, if only to cover my own ass. Like if the picture wasn't clear but you accepted it anyway, you had reason to know? Seems pretty loose for anyone that wants to enforce it.
I'm clearly not an attorney, so if you are, please correct me.