I think you might be confusing "fires low performers" with "fires people who have occasional misses." There's a world of difference between the two.

I have a few friends who I worked with at a large company, the kind with a large bureaucratic, broken management system that shuffles around incompetence without ever purging it. They went on to work at Netflix and they tell me how much they love it. They no longer have to deal with terminally incompetent divisions. They're top performers; there's no fear. If Netflix were stupid enough to fire them they'd instantly be buried in new opportunity. Of course, Netflix isn't that stupid.

Consider the economic incentives: A strong will-fire policy is a great negative incentive to prevent incompetent hires in the first place. It's not so scary if you're competent and capable -- and that's precisely the kind of people a company will want to hire..