Because unless you're using an MVA (Convertacom), the standard charger is not designed to handle the load draw it will encounter when the radio transmits and puts a draw on the battery. It's just not a good idea, and there's really no reason at all to be transmitting with it in there. Anytime I see someone grab the speaker mic and key up without pulling the radio out first, I cringe. If the battery isn't in great shape, or it were to fail, there's a real chance of the current output from the charger spiking way high and surging right through the battery and into the radio itself. 9 out of 10 times you'll probably never have a problem, but why take the chance? It's just sound advice not to transmit with the radio sitting in the charger, unless it's a charger designed for that type of operation.