I'm gonna guess that the requirement is there because there's more at stake - if the iPad or iPhone lock up during an update, there are ways to recover it on your own. If the Apple Watch locks up during an update (like, say, from power running out), you have to send it back to Apple.The only thing I hope is that they remove soon the requirement for the watch to be on its charger. They don’t require the iphone and ipad to be connected to power for updating, why do they require it for the watch?
I'd like to see the restriction removed as well, but I can imagine that's why it's there. In any case, I downloaded the update directly on my watch, then went back in and it said it had to be on the charger - put it on the charger for about 5 minutes and it updated. Not a big deal (unless you don't happen to have a charger handy).