Stack Ranking seems like it should be a perfectly legal method of performance management. You might choose to not work at a company that practices it, but why should it be illegal?
It is perhaps a natural fear that a Stack Ranking system would lead to a "Rank and Yank", but the mere ordering of employees during a performance management process doesn't give me any particular concern, nor do I consider it a bad faith system.
In fact, it forces people to make a sometimes hard choice, which I believe has more benefits than drawbacks to an organization, else you run the risk of having a massive "undifferentiated middle" and no clear development plans because you have no feedback from rich discussion about why Jane is better than Joe; instead Jane and Joe are simply declared to be equal without any discussion about what Joe could do to improve or what Jane should continue doing, etc.