Scale. Pointing at any one retailer and saying "that is scummy, stop it" does not work, feels arbitrary and "then go elsewhere" is actually an option. If Amazon or a number of large chains do it, there is no reasonable alternative.

Case in point from Germany. If small supermarket chains treat their suppliers badly, that is their business. If Aldi (very large chain) does, this is potentialky ruinous for many companies, so politics do get involved.

It always is a tradeoff between the possible harm, number of people affected and the desire to not intervene if at all possible.