Employment is at-will but you can’t fire someone for non-work reasons. The PIP is there to collect the data that they are firing someone for work reasons.

Maybe I need my understanding corrected. I thought the point of at-will was that you very much could fire someone for nearly reason (you support the Cubs, pack up your desk!). You cannot fire someone for protected classes (gender, race, etc), but basically everything else is fair game.

Who said anything about firing for non-work reasons. One can be fired immediately for work reasons without a PIP.

Note that the PIP alone does not suffice as valid supporting data if the employee contests it. It helps for there to be more data than it.