You don't just stop after what you're tried doesn't work. You fix the problem with EO's, legislation or change of strategy. You raid Tyson again, for other counts. Just because you got out of one count of an illegal act, doesn't mean you can't be prosecuted on other counts.
The case fell on points that could easily be fixed. Goodness. The claim that the top management didn't know what their manager were doing would be mooted in the next case, since they had been warned.
Not to mention trying to prosecute several companies for conspiracy! Do one, get the others if they haven't changed their ways. I don't buy it. If the executive wanted to crack down effectively on illegal hiring, it would.