The reason you are asked not to simply charge your device is because of the potential for a fire to start if someone is using a third party usb cable/plug which can be prone to failure resulting in dangerous over heating and in some cases combustion. . This is particularly the case if you've bought the very cheap cables online from the likes of Amazon or eBay.People often go to sleep, or put the device out of sight in a seat pocket or drawer, whilst charging it which means it is unobserved.