What prevented them from trying again? I doubt the settlement had a clause that said "you can't start a union".
Why wouldn't it have one? I'm sure the lawyers involved here wouldn't miss the obvious question. Seems like a sound way to make the issue go away for a while: pay the instigators a bunch of money with the stipulation that they not continue to instigate.
Nothing stops other people from trying again, but buying out the ones in charge of the effort goes a long way towards setting it back. Plus, by settling out of court, the company avoids the risk that the court decide its behavior was illegal. The courts may do so in the future, but it would take getting caught again. In the meantime, the company can refine its methods of limiting unionization while better avoiding obviously illegal behavior.
Most of that is contingent on setting existing unionization efforts back, to buy time for efforts to be mobilized or improved. The easiest way to set any social movement back is to eliminate key points of coordination. Putting an NDA and other clauses into the settlement agreement achieves that end and puts the law on the company's side.
All that's left at that point is to figure out everyone's price.
Likely the illegal bits were whispers, winks, and head nods.
Because that's illegal?
And a no union clause would be just as illegal as their conduct in the first place.
I think the problem is that unions can be quite hard to create to begin with. Lots of people have to stick their neck out, and often lose their career. Not that many people at any given company have much power. Most people are trying to make ends meet.
The sum wasn't insubstantial, but it was much less than the pay that would have been received in a free market.