Really? It sounds like you're describing a felony, and one that would probably not that hard to be traced back to the perpetrator.

You're naive if you think this doesn't happen more often than not. Some shops are very good about preventing this, most enterprises are not. Drop a bunch of USB sticks in the parking lot or other common areas, someone else ends up doing the work for you.

I do incident response for a living. Disgruntled employees/former employees do sabotage their companies' networks, but it is incredibly rare. Not because most organizations are well-defended against such attacks; its because most people aren't willing to destroy their lives simply because they were laid off. Especially in a rapidly growing industry where a talented person can generally find a better job than the one they had in a week or two.