Really?
The engineers figured out that they can use the 6 pin diagnostic port to charge the watch. Since it is a diagnostic port, similar to let's say the Lightning port or a USB, signals (including electric current) can travel both ways. I would think that the engineers also know that the watch can also be used to charge the band. Obviously there is more to it than that, but that's the basic idea. It's really no different than a cell phone. The battery in the phone powers the phone, but the battery is also charged by the phone. They are just adding additional battery.
I've been thinking about this some more. Imagine for example this strap does have a connector which recesses into the strap when you plug and unplug it; I can see that it would be quite flimsy and prone to breaking. I expect you would need to keep removing it in order to charge the strap (charge the watch in the usual way) otherwise you would need yet more ports on it somewhere to charge the strap itself. Constant removing of the strap would fatigue the mechanism and over time the connector would work loose, not make proper contact and causing problems. I also doubt it would do the watch's own strap mechanism much good either.
The engineers figured out that they can use the 6 pin diagnostic port to charge the watch. Since it is a diagnostic port, similar to let's say the Lightning port or a USB, signals (including electric current) can travel both ways. I would think that the engineers also know that the watch can also be used to charge the band. Obviously there is more to it than that, but that's the basic idea. It's really no different than a cell phone. The battery in the phone powers the phone, but the battery is also charged by the phone. They are just adding additional battery.