Leaving aside the amount of money union vs. non-union labor costs, its not surprising that employers oppose unionization because it constrains their flexibility on labor. Makes it harder to fire or restructure workers and creates labor uncertainties around strikes.
Still blows my mind that its cheaper to throw these sweatshops up around Asia and train a workforce than it is to simply redesign the iPhone to be easily assembled by an automated assembly line. Car companies automate far large and more complicated assembly processes than a phone.