This would be because it takes the tv unit out of the equation if something would happen during the charging process. Though it's rare for this unit to overcharge it's self, you wouldn't want the unit to be accidently turned on during charging as you may short the tv out. This wouldn't be covered by warranty either. I've never heard of someone charging a battery while it's connected to a supply unit, not sure why this would be any different other than being inconvienent.
Tight Lines,
GatorBait