Amazon has a policy of binning items with the same UPC together, regardless of the source. What this means is if you buy a valid product and any vendor who is part of their warehouse storage system sells counterfeits, then there is a chance of you getting a counterfeit part, regardless of who you buy from. This reduces the number of locations required for a given item. It just requires that you trust your vendors to not counterfeit. If they were kept separate you could easily see who is selling counterfeits, but it would require more space.

So Amazon has traded the ability to sell parts from verifiable vendors for short-term profits. At this point in the game, your best assumption is if there is any knock-off company selling the product you wish to buy you have no way of knowing it it's legitimate or counterfeit. This is currently diluting their brand and will ultimately impact their sales, if not their profits.