Anyone using Google voice commands has agreed to let Google listen constantly for instance. In the EU there are restrictions on what they can do with the data, but the collection itself is perfectly legal and happening now. How else does your phone/nest know you said "OK Google"?
As I understand it, Google currently don't bother as your search history, purchase history, emails and location data all do a better job of predicting what you will buy than voice. Plus those are easier to process. But yeah, they can.
Edit: I guess Amazon are more likely to be doing this with voice data from Echo as they don't have your email/location etc the way Google and Apple do. I think they're more tight lipped about what they do though.
I think the USA recently tried to bring in some restrictions on tracking and compiling data (not just voice) on under 13s. But this is the internet: everyone is 18 apparently and it's honestly not possible to check or enforce on any scale.
Check local wire tap laws. It various everywhere.