The odiousness of the use of the word "should" has been pointed out elsewhere, and I reiterate it, but there is another point to be made about this analysis: It neglects that Amazon is a monopsony or near-monopsony. As Amazon replaces other supply chains, the demand for supply-chain labor is increasingly concentrated in one buyer of labor, Amazon. We should reasonably expect a distortion in the market to occur wherein Amazon leverages their power to either reduce wages or demand more labor for the equivalent wage.
In effect, the market conditions to which the OP appeals are eroded away by Amazon's cornering of this sector of the labor market. $15/hr may be good PR, but it's not a "fair" or "market" wage if Amazon can just burn through employees with adverse working conditions (ostensibly valued at a higher rate in a real market) because of their position as the sole buyer of labor.
If we're going to apply market theory to this, we have to go all the way, and not just stop when it suits our purposes.