For those wondering why?
"The Battery Status API can be used to defer or scale back work when the device is not charging in or is low on battery. An archetype of an advanced web application, a web-based email client, may check the server for new email every few seconds if the device is charging, but do so less frequently if the device is not charging or is low on battery. Another example is a web-based word processor which could monitor the battery level and save changes before the battery runs out to prevent data loss."
That seems reasonable; no one wants a Windows 10 update to start just as a battery is going flat, and, just as it would be nice if local apps could take account of battery condition, why not the same for cloudy-based apps?
As long as there's an 'off switch' client-side or a means to override what is sent I don't see there's really a problem.