The value is in Amazon's voice services and speech recognition platform, not in the Echo device itself. As machine learning improves, voice and speech may play a bigger part in user interaction, and Amazon wants to be out in front of that.

The Echo hardware was just a way to get the ball rolling with this platform.

I'd imagine the incentive is more points for the speech recognition model. The race is for who can handle completely unstructured speech best, and the field is vast at this stage.

A direct response to Google opening their Voice Recognition API?

Like with their hackable instant order button, I would expect that at least for the short term, they want to use it as a way to make purchasing as fast and easy as possible, obviously to maximize their income.

Longer term, conversational AI is clearly the next huge thing in sales and customer interaction, and they probably want to start progressively building grounds in that domain

Guessing they'd make more money off of people ordering products through any echo device than they would on their own hardware?

Hypothetically, amazon wants to make money of what people use echo for and not the hardware itself.