You make the assumption that all management is bad and all unions are good, niether of these statements is inherrently true.
Unions are democracies. Corporations exist only for one purpose: the almighty dollar. It isn't necessarily that management is "bad," although certain management certainly are, it's more about who they are beholden to. Unions have a legal responsibility to their members. Corporations only have a legal responsibility to their shareholders. As an employee, it is asinine to put your trust into someone that has a fiduciary responsibility to someone that has goals antithetical to your own. The shareholders want to see labor paid as little as possible to increase their future profits. Therefore, management will do what they can to make those goals happen in order to pacify the shareholders. Unions exist to provide balance. Capitalism run amok is one of the scariest notions that I can imagine. There has to be balancing forces.