In practice, it's usually not difficult to come up with a legitimate reason to fire someone. For example, there is a whole industry of computer forensics consults who for a relatively small fee will "probe" (search the target's work computer and network logs for things that shouldn't be there) an employee to come up with a legitimate reason for firing them.
Reason is that if a person is fired than you are a huge, rich target for a lawsuit. Each and every one of your employees is now a liability. Someone say an off-color joke or do something that can be even remotely considered discriminatory and you're on the hook. I've had colleagues get rightly fired for doing incredibly stupid things. They lawyer up and bring a suit about some manager being racist, sexist, or religious insensitivity. They know the company would rather pay a few thousand dollars than have to pay $500/hr for a lawyer.
If your company is viewed as discriminatory towards employees (doesn't matter what the facts are) you'll be paying more money to recover your reputation. So severance and settlements are much cheaper to the bottom line then trying to convince people of your story.
However, the U.S. is also highly litigious in the civil courts. If you feel you've been wronged, there are a multitude of ways to bring up a lawsuit... and plenty of lawyers who would be willing to take your case (especially with "tens of millions of dollars" of denied stock options on the line).
The factors that can't be considered are 'protected classes':
http://en.wikipedia.org/wiki/Protected_class
So when a decision based on one of these classes could be alleged, employment changes/terminations can become riskier.
There's no right to continuing employment, or direct severance/buyout payments, as seems to be the case in some European jurisdictions. But when more of your employees wind up collecting state-administered unemployment compensation, your payments into that program escalate. (Anyone not fired for specific misdeeds is eligible for unemployment compensation, for a limited period, while actively seeking other work.)