To protect you, your computer and your Dura/Kissbox or Stick from electrical accidents!
Could you explain that reasoning in a more detailed manner? It seems to infer that anyone who uses their usb charger on their e-cig battery is taking a chance they shouldn't. I also wonder what the effect on the battery in your passthrough is when it is being charged constantly. Also, since there is a battery in there does that mean the device can be used when you aren't near a computer?