USB is a supposed "smart" protocol, technically USB "gadgets" aren't valid devices, i.e: devices without data lines.
Power usage is negotiated, without this negotiation you're really only permitted to use 100 mA, but some devices pull the maximum of 500 mA ignoring the rules.
Two USB cables allows the device to use more power, although it's sort of cheating if one of the cables is "dumb", as described by ocicat.