What am I missing from this?
One example: my Nexus 7 draws so much power, even when sleeping, that it is possible to connect it to a weakly charging USB port, come back a few hours later, and it has a lower charge level. I'm sure the same is true for other tablets, and possibly even some phones.
there's a widget that will let you disable wifi, bluetooth and GPS from your homescreen. turn those puppies off before you sleep, and it'll charge in no time.