However, IF the stories are true about the employer threatening workers with calling immigration, then they know many of their workers aren't legal. You do however need to prove they knew individual workers were illegals, not just that some had to be illegals. Such proof is going to be hard to come by, so it will be possible to prosecute very few.
If the employers are clearly exploiting the workers' illegal status, then those employers should not only be facing fines but jail terms.
However, the mere presence of illegal aliens isn't evidence of guilt since the government does make it very difficult for employers to verify the identities and legal status of employees.
The government has an E-Verify program. Does all the work for you.