If you're a manager in a situation where an employee has a problem and aren't meeting the job requirements, you'd probably much rather that they fix the problem and contribute to the team than fire them.
That doesn't mean that all of these plans are done in good faith, or that some managers aren't terrible. And for an employee on a PIP, they should think hard about leaving the company (or at least the team); it'll be better for their career.
But I don't think one can say that it's nothing more than a legal way to fire someone (especially since you can fire someone in most states in the US without cause, and spurious allegations of racial or other discrimination would need documentation that would be hard to produce if it didn't actually happen).