I can understand the SDK being free, it would seem strange to me that a commercial product could actually plug in to the service for free, and without limits.
Ultimately, the service enables people to order from Amazon and gathers training data for their algorithms, so regardless of whether you're using it through an Echo or a Furby or whatever else, they win.
To put it bluntly, why would they stop others from making their profitable data collection easier and doing it for them?
I'd not be that surprised, if when you build with it it has the same kind of features as an echo. Wanting to play music means you are more likely to get amazon music prime, or pay for song storage. Ordering directly and making shopping lists pushes you towards amazon.
It may be cheaper for them than the affiliate scheme.
To put it bluntly, why would they stop others from making their profitable data collection easier and doing it for them?
https://en.wikipedia.org/wiki/Amazon_Echo#Privacy_concerns