It is a good question and in an isolated environment, it would be a great solution! But those concessions are usually in contract language. As well, they are on an entire invoice, not just an individual product. Well, they could be on an individual product, but usually because of quantity! If you, as a reseller, buy a huge bulk, you expect a better discount right? Better than your normal discount.
Here in the US, it works out kind of weird. You would think a book publisher would have a lot of leverage with a reseller, any reseller, right? After all, what is the reseller going to do? Turn down the opportunity to have the New York Times Best Sellers in their store? (This is one argument I have heard on why publishers should dictate conditions. If the reseller doesn't agree, the reseller doesn't get to stock any of your product.) Except for one problem: It is a best sellers list! Publishers don't dictate the list, the consumers do! If you don't have your book in every Barnes & Noble, it will not be a best seller on the New York Times list. Because the big resellers were initially successful, and grew in size, they now grossly outnumber the independents. They have for years. Some of that growth likely came at seizing opportunities. Many businesses stumble once in a while. Publishers sometimes make a bad call on a book and a print run. When a big company turns around and helps the publisher through that stumble, that company then has more leverage to win concessions. After a long enough time, those concessions become normal behavior.
Sure, Malhavoc and White Wolf could tell the book distribution channel that there will be no special pricing or other concessions. The book channel would likely tell them to stuff it. From the distribution channel's perspective, publishers need them more. Especially if it is a small publisher. As I said earlier "The loss of our sales to the big chains is less than a rounding error in the context of their overall sales." When the bookstore I worked at stopped selling gaming books, it wasn't even noticed in our sales. *shrug* Nowadays I see WotC books back in the same bookstore. But I don't see other publishers. The book distribution channel can work for some companies. But how many of those are in the hobby right now?