This is about charging, not how it sends data. And the batteries last a long time for this particular device, so there's minimal downside to unplugging. The downside to staying plugged in is there's a cable dragging around that makes the experience of moving the mouse a little bit worse all the time.

A user has to be thinking about something to make a choice. Depending on the user to think about all these little aspects degrades the experience all by itself.