Yep I agree, and just to be thorough, it's also possible that unions are good for management and bad for employees (let's say the mgt and the union bosses collude to split the difference and shaft the workers), or good for both management and employees (for example better health plan, lower turnover, lower absenteeism, better morale & customer/client service and ultimately greater market success & higher profits).

There's no particular reason to assume out-of-hand without further information that the interests of employees and management are intertwined (basically my earlier point), nor that they're separate & opposed (basically your point). But management by opposing the union so strongly is implicitly asserting that at a minimum they believe it'll be bad for management. They can also be wrong vs. right in that prediction. They may also explicitly assert it'll be bad for workers, and be wrong or right in that. Seems like there's a lot of game theory happening here and I bet there's some concise notation for all this. (Not my strong suit.)