Another reason to charge the lipo battery outside the radio is due to the balance lead/charger. In order to properly charge a 3-cell lipo it should be done on a balance charger which has a unique connector not found on the radio.

Most likely, the 3-cell lipo will have a 4-wire Hyperion or Align connector on it which you must plug into the charger. Not only will charging through the radio not completely charge each cell in the 3-cell pack, but it is also dangerous as you can't observe the battery.

If the battery starts to swell while charging or even get warm.... RUN!

Well, more seriously, immediately stop charging and disconnect everything as a warm lipo or a swelling lipo is a bad sign.