They could, in theory, have contracts that say the AI can't fire them.

It could be set up such that the AI can "fire" them, in that they no longer work at the store, and aren't paid wages that count against the experimental establishment's costs, but still get paid to do something else, or to do nothing at all.

I doubt the experiment is set up that way, but that would be an ethical way to do it.

There’s no way they are putting that into a contract. HRs are already using it to fire people.