It would only be reasonable to do it if they have a very lower power way to monitor for it all without being plugged in, and if there's some sort of voice recognition is built in so that it wouldn't just take any random person saying "Hey Siri" or even misinterpretations of something from TV or elsewhere that the phone interprets as "Hey Siri". Right now neither of those are a reality (yet), so it makes sense for it to be the way that it is.I do know the definition of counterintuitive and meant to use it, but thanks for your failed attempt to enlighten me.
There are several Android phones that have have a 'OK Google' listening mode while NOT plugged in and it makes sense if you like the feature.
I use Siri during the day while unplugged but I don't use Siri at night while it's plugged in charging: counterintuitive