In California, one gets laid off (not fired, but laid off) after not passing PIP. One can also get laid off "at will" - any time for any reason.
When you first get a job, you sign an employment agreement that outlines your responsibilities. If you're actually doing a poor job, why would the employer bother putting you through a PIP? They'd just lay you off. The PIP is a renegotiation of your employment without giving you a raise. They'd like you to do more for less because they think that either the market or your financial position is in their favor.
PIPs are mostly used so they can get rid of people without fear of a lawsuit. It generates documentation that “hey, we tried” in case it’s needed as evidence later.
When you first get a job, you sign an employment agreement that outlines your responsibilities. If you're actually doing a poor job, why would the employer bother putting you through a PIP? They'd just lay you off. The PIP is a renegotiation of your employment without giving you a raise. They'd like you to do more for less because they think that either the market or your financial position is in their favor.