A USB charging port can have some active circuitry to do things like sense when something is plugged in, what the charge compatibility is, etc., That being said, I can't see how that alone (ie. not using the light while having a battery inserted) would drain a battery in just a couple of days unless it was defective. I can't see how/why the lamp part of it would have a circuit that would be active with the light off at all.
