Workers that fight for their rights and form unions and what else aren't very efficient.
That's why a lot of industries moved to china or other poor countries (now they are moving AWAY from china, since it's no more so poor), to exploit their cheaper workforce with less rights.
All people in a marketplace seek the lowest price for a given good. You don't pay more for bread than you need to. A company will not pay more for labor than it needs to.
So long as a corporation is not enslaving workers at its factory (preventing them from leaving if they so choose) then there is no moral problem - and indeed one should realize that the corporation is providing the best option for employment. If a better option existed, the workers would pursue it.
You use the term "exploit" - but companies aren't exploiting workers any more than you exploit the baker whose bread you buy.