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The theory of surplus value isn't really about fairness. It simply says that there must be some value above the "socially necessary labor time" required to produce commodities in order for the category of profit to exist. It's a critique in the sense that it takes the surface-level appearance of concepts like value and profit for granted as they are, then sort of reverse-engineers them to illuminate their conditions of possibility and the relationships between them. So surplus value isn't the same as profit, and in later stages of the analysis it's argued that for example "aggregate" surplus value can be transferred between industries based on those relative rates of profit without any "real" transfers taking place. It has been a little while since I really had a firm hold on the theory but if you're interested in how it's applied to describe conditions of overproduction and overproduction crises, Ernest Mandel's Late Capitalism has a good analysis of the 1980s.