I don't know where it stems from, but I think it's harder to fire people in union shops. In a non-union shop, you have to work with the person, their manager, and HR to see if you can correct someone's performance, and if that does not work, let them go. At a union shop, I imagine someone from the union will also be involved in this process, and it's hard to imagine how this process would be made easier through the involvement of another person, who happens to hold a bias.