Incidentally, I don't know why labor unions are associated with leftists and Marxism. They help the market determine the correct price of labor.
Labor unions are associated with leftists because, while the IDEA of a labor union supports free market bargaining, US laws provided expanded legal support for unions at the expense of big business. It's not simply collective bargaining, which would be fine--it's legally mandated union time, rules about bargaining in good faith, etc.