As for Echo, the microphone is the primary purpose of the device. Home automation and ordering by voice require a microphone, not a speaker. The Echo Dot even includes a line-out port to connect to external speakers, in which case it isn't acting as a speaker at all. It doesn't include a line-in port.
Speakers don't: answer questions, set alarms, turn on lights, adjust thermostats, lock doors, call almost anyone hands-free, drop in on other rooms in your home, make an announcement to every room. (all advertised features of the Echo)
So following the convention you mentioned, it's much more a smart microphone than a smart speaker.