If Burger King significantly increased the salary of their workers or otherwise found ways to improve their morale, would it benefit Burger King in the long run? I tend to suspect so, but it would be hard to prove.
The burger joints known for taking care of their people are also usually seen as being pretty good burger places, and that may be causal, but it also may be that they're just slightly "upscale" of the other burger places and also spend more on the ingredients.