My experience is similar, though I would personally attribute it at least partly to stupidity - on the one hand, in most cases when I have worked with someone crucial, they've been with the company longer than the people managing them, so the person or people making the firing decision don't really understand what's at stake. Or to put it another way, their cost-benefit is flawed, they know what it'd cost to keep the key employee, but they fail to imagine what it will cost to let them go.