How do you exploit someone who, according to you, will "just walk away" if he doesn't feel he's being properly compensated? I suppose you could, maybe, possibly exploit an employee in some rare circumstances. A Ground contractor could probably try to exploit his employees, and he might succeed for a brief period until he realizes that exploitative practices hurt his bank account.
As you pointed out, substandard wages create (among other things) turnover. Turnover creates additional expenses that eat away at the bottom line. Too much turnover is a sign that the wages offered aren't competitive enough.