It's much easier (maybe too easy) for anyone to sell on Amazon compared to Walmart.com. This allows Amazon to alway have a huge selection with a bunch of sellers competing to offer the lower price. This is going to let Amazon stay in front until the other companies catch up but it might be too late by then.
Walmart has a third-party reselling program now too, and they recently acquired Jet.com, which also has one. Most big online retailers are getting one because it's very lucrative.
FBA is not unique, it's just the best known. Amazon's laxness is coming back to bite them, as more and more stories proliferate about the receipt of counterfeit Chinese goods from "official" Amazon inventory (due to commingling).
At some point, and they're already working on getting there with their opt-in brand controls, Amazon going to be as restricted as other retail outlets. Legal intricacies around trademarks, warranties, merchantability, and the meaning of a "New" product make it risky to allow any "unauthorized" distributors for anything that people would want to buy.
