I think the idea is that it allows for Amazon to remove some strength from the official wholesaler side. If the wholesaler isn't the only source of goods they have less bargaining power. But at the end of the day the wholesaler has to sell to someone. Here are my thoughts on how this relationship could work to Amazon's advantage.

Think of this scenario:

  **Wholesaler**: Give us $10 per unit to list our 
  products on Amazon.

  **Amazon**: No, we'll pay $15 per unit from a 3rd 
  parties you sell to, but we won't tell you who.

  **Wholesaler**: Whatever, we'll still make sales at the 
  end of the day.

-- Some time passes --

  **Amazon**: Hey 3rd parties, if you store all of our stuff in the warehouse we'll be an awesome partner and increase your profits a bit!

  **3rd parties**: This is an awesome deal!

  *3rd Parties* who are unaware of the fight between the 
  wholesaler and Amazon sees their business going great 
  buys lots of stock from the wholesaler, sends it all to 
  Amazon, has a lot of their sales going through Amazon, 
  etc.

-- Some more time passes --

  **Amazon**: Hey Wholesaler, we have these 3rd parties 
  and 90% of their revenue is through our platform. You 
  are going to list your product on our platform at $5 
  per unit or we're going to either A: Delist the 
  products and kick the sellers off our platform and/or 
  B: We're going to use our business intelligence to 
  launch AmazonBasics™ versions of your product(s) and 
  throw your customers to the wolves while your demand 
  crumbles into to dust.

Today YOU are here. This is how they use their monopoly power to get an unfair benefit over the existing manufacturers/wholesalers. And they can use 3rd parties, small businesses, as a way to get leverage. These 3rd parties will go and get business loans, they'll build up their inventories, they provide an abstraction that's hard for the wholesalers to fully peer through to, etc. These 3rd parties are just a pawn in Amazon's games with wholesalers. For every dollar Amazon burns giving these 3rd parties more money than the wholesaler they're really just leveraging indirectly the credit of these 3rd parties and throwing them to the wolves if they don't get their way. Simply a pawn in a game for Amazon to consolidate more power.