All I can tell you is that they aren't doing the whole 3rd party marketplace out of the goodness of their heart.
What better way to do market research than to pit tens of thousands of individual resellers and manufacturers against each other to determine demand and equilibrium prices of the end consumer?
Then you just private label it and cut out all the middlemen, and sell direct. There's no way they can now compete with you, because:
1. You're Amazon, so you have the brand behind your product
2. You're Amazon, so you have efficiencies of enormous scale (in both manufacturing and distribution)
3. You're Amazon, so you can actually kick them off the market and delete their listing for any bullshit reason you can put in your TOS
Perhaps this was the plan all along...