Why is it that capitalism's piece-rate wage is also a form of exploitation?

Why capitalism is essentially exploitative is this. The capitalist pays the worker a wage that is less than the value that the worker creates, and that creates exploitation. For example, if the raw materials and other costs are 10 dollars, and the cup is sold for 20 dollars, the value added is 10 dollars of the worker's labor. The value-added of this ten is created by the labor of the workers, the labor creates the value of ten dollars, and the capitalist will only pay the workers six, the rest of the four (surplus value) is exploited by the capitalist away.

The capitalist can never pay the worker the full ten dollars, so there is no way to change the fact that the essence of capitalism is exploitation.