I think you're right about such a policy being traditionally applied on retailers as a means to pressure the MFGs, but in Amazon's case, I believe it's different, because it's Amazon itself who's name on the product; they caused it to be built.

Retailers, as you say, can also be sued for infringment since they are in the chain of commerce, but typically they do not cause those products to be produced, but simply create the means of distribution to the end user. I think this is mainly where your observation of using the technique of pressuring the MFG via the retailers works.