As they've implied in The Godfather, taking things personally is bad for business. Companies that make money do whatever is necessary to keep the flow going. Revenge is a detour to this process so unless they're trying to stifle competition or the like, destroying someone isn't worth it.
People who are in revenge mode don't care what's good for business. It makes them feel better and that matters more. In the case of suing former employees, it also sends a clear message to existing employees.
More often than not, executives/companies think they're doing you a favor by giving you a job, even the ones who pretend to care about their employees.
Your post puzzles me as well. Just because we use the word "giving" doesn't mean that any of the connotations of that word actually apply to the situation. Employment is an arrangement of mutual benefit. In some cases an employer might give a job to a friend as a favor, when the friend is not the most qualified for the job. In any case, if someone gives you a job as a favor, they are doing it wrong.
That's how I look at it, but I've encountered way more employers (across multiple industries, in companies of all sizes) who have the mentality of "people should feel lucky that they have a job with us". This attitude is even worse during an economic downturn.
Revenge is bad for business.