Does this mayhem actually ever happen or is it paranoia? I have seen several layoffs where people got advance warning of a few weeks up to almost a year. I have never seen people do anything bad once they got told they would be laid off.
Other than general goodwill, there is no incentive to give an employee a heads-up that they are being laid off. And lots of downside if the employee happens to be disgruntled and do something evil. Goodwill does not show up on balance sheets. But theft and sabotage do.
I'm not saying I like it, but I understand it.
"The value of a company’s name, brand reputation, loyal customer base, solid customer service, good employee relations, and proprietary technology represent aspects of goodwill. This value is why one company may pay a premium for another."