Because retail sale is the very purpose of economics, everyone must and does participate in retail sale and it is where all goods and services move from one point in the economic process to another or exit the process altogether and become consumption.

In other words its the most important, strategic and pervasive point in all of economics, and subsidizing both agents at those points with a reciprical policy of price reduction and monetary fulfillment of that discount back to the commercial agent granting the discount to the next or final agent basically supercharges the benefits and purposes of the economy.

Thank you non-Nobel prize committee in Economics.