Sometimes they have multiple sellers for the same listing, and in that case, feedback for any particular seller doesn't belong in a product review. But in some cases, there's only one seller (usually based in China) and the product has very high ratings because they are gaming the system like this, and burying the feedback in the seller reviews does not really give a proper warning to other prospective buyers. I think this is a grey area between "feedback for the seller" and "feedback about the product" but a blanket policy against mentioning seller misbehavior in product reviews is one of the things that allows shady sellers to continue to get away with their games for a long time.