Because people deserve rights in the workplace. You should be able to hire and fire who you want, but for valid reasons, in no way should you be able to fire who you want for whatever reason you want. At least not IMO.
Like I said, if you offered me 2 exact same jobs, one with a union presence and one without, id take the union one everytime, becasue my experience is those employers treat you like a human being, whether its forced or not. The same isnt true in my experiences of non union workplaces, where they are out to screw you for everything they can.
At the end of the day, an employee-employer relaitonship is supposed to be mutually beneficial.