No one is saying that every single employee at the bottom is being exploited - just that exploitation is rational for those in power, because there's no particular incentive for them to completely avoid it.
They shouldn't do it too much, or then society responds in various ways (unions, legislation, etc.), so in that sense it's much like shareholder value. The company owners cannot write themselves a bonus equal to the entire profits of the company, or the shareholders will get mad. But they can certainly write themselves generous bonuses nonetheless. They don't have to completely maximize shareholder value, or completely minimize exploitation; they just have to do enough.
