By providing the exchange, AND holding a good deal of the currency -- they could buy up 40% of the bitcoins in existence easily right now -- they could actually make the market and hold prices more stable just by selling their own reserves during runs and buying during crashes.
Yes, they already have Amazon coin, but why not take advantage of the proven protocol of bitcoin, and the widespread holdings of / interest in the currency?
I agree with the second part of your post, and the first part, while it does provide increased stability and may increase the adoption of bitcoin severely, I feel it goes against bitcoin having centralization. Though, I am on the fence about that. Stability would most certainly be extremely welcome right now.