The purpose of the "Hey Siri" and "Okay, Google" functionality is to listen to those specific phrases. It is not intended to be used to covertly record people and can not be used by a regular consumer to do so. The law does not concern itself with technicalities such as what constitutes an "active microphone". What matters is intent. That's a pretty easy question to answer for those watches. This functionality is literally a bullet-point in their ads.
And yet we've seen similar technology in other products, like Samsung TVs, being used to gather huge amounts of information for the U.S. government. From a security standpoint, understanding the capabilities is really all that's relevant. If it's possible that it could be used in that way, then it probably can't be trusted not to be used in that way. Especially since these devices receive OTA updates and there are a number of ways to attack them on a relatively large scale by doing things like spoofing a cell tower.
If the concern is individuals using their own devices intentionally for spying purposes...that's relatively easy for a non-technical person to do with a smartphone if they want to.