"Potentially" but we can examine network activity and verify that it is not in fact sending data all the time.
If I were planning to use these devices for surveillance, I'd of course provide only the utility first while privacy-minded people are still skeptical and then turn on the surveillance gradually after these devices are deployed nearly everywhere, integrated into every-day life, and can't be (easily) removed anymore. [0]
The current version might have a hardware kill switch for the microphone. Will the next batch too? The batch after that? How many people will actually go out of their way to toggle that switch to be certain it isn't listening instead of believing it isn't in good faith, or worse, not even thinking about whether it might?
Perhaps these devices won't ever be used for mass surveillance, or perhaps this is the beginning of Telescreens as they're described in George Orwell's 1984. Both Amazon and Google make vast quantities of their income by excessively undermining privacy in favor of targeted advertising. I'd call it fairly naive to expect them to now build privacy-minded devices that have as much spying potential as the Echo and Dot have.
[0]: https://media.ford.com/content/fordmedia/fna/us/en/news/2017...
Wouldn't that apply to (mics in) smartphones as well?
Keeping the recent and current state of technology and where it is heading in mind, I think it's fair to say that consumers care a lot more about being able to use their phone than they care about that phone or the apps on the phone to collect data about them.
I have seen more than one comment suggesting that Amazon currently stores everything it hears in passing on their servers. I have seen no evidence to suggest that is the case. Informing people on the potential risks is fine, but spreading FUD is not.