I have an Echo. It's great. To me the Amazon play for this is very straightforward: they want you buying Amazon stuff using it. They don't sell ads, they sell products. When they say that the device only listens for its own name locally, I believe it (sans someone hacking it to spy on people of course, but I don't believe that Amazon deliberately programmed it to do this).

Google is a different story. I imagine that they would try to sell a device that listens 100% of the time and targets ads at you based on what you talk about. Google makes money by selling ads, not by making buying stuff convenient. Their incentive is different.

Ideally, I'd love for Apple to create such a device, but it won't be open, would cost an arm and a leg, and would probably use Siri, which lags behind the Echo in what it can do by quite a bit now.