Back in the late 90's, early 2000's before Amazon was as big as they are now, I worked in the commercial music distribution arm of Sony/BMG. We were the ONLY people that were manufacturing (In our own facilities) CD's for bands on our music labels and yet bootleg copies of CD's were going out Amazon's doors to customers. How? Amazon doesn't keep an abundance of stock on hand. If they run to low, they would buy product for One Stops (at a higher price than we sold to them) because the One Stops could get several thousand CD's in a few hours while an order from our warehouse might take 36 hours to get there. It was a stop gap for them to not go Out Of Stock. Those One Stops should only have been getting their product directly from us also, but no one watched their inventory as closely, and they would at times buy product from unofficial manufacturers. Those One Stops were trying to scam people. (Mostly Mom and Pop CD retailers)
Again, I have not had any problem returning product to Amazon.
Have you got any source for this being a real problem ? Because it seems such things are easily detected at the packing station, by barcode. And if an employee does so often, he'll probably won't stay long.