Looks like amazon might be afraid that Dropbox has plans to potentially build their own server infrastructure.
> It's not so much a hunch as it is common sense
So you are completely right, he cited no source.
But even for smaller amounts, if you sell 150K of goods per year through a reseller, and you pay a standard fee, it never hurts to e-mail them and negotiate a tighter deal. If you manage to shave off 1%, that's $1500 (clean profit). Say you pay 5%, you can apply pressure by saying you could get a better deal somewhere else, or implement their service yourself for that money.
Bezos made a decision to not negotiate with individual customers, and there are pros and cons to that decision.