Specifically regarding at-will employment and the United States, you can still be sued for discrimination based on race, gender, and all other protected classes. PIPs and other similar processes are primarily created, from a legal/HR perspective, to protect against those lawsuits.

So, somewhat counterintuitively, at-will employment in the United States(especially in states like California) is not actually at-will in a practical sense do to the threat of discrimination lawsuits.

Funny how they are used as a cover to those lawsuits but at the same time anyone with half a brain can see how a racist could saddle someone with bogus pips. Someone must have brought this shaky pip defence in front of the right corporate favoring judge if everyone is using it as this ironclad defense now.