The engineering of a battery powered device requires power consumption to be kept to a minimal. This criteria becomes less important in a plug in device as a result the engineer of the device is less inclined to ensure that it consumes as little power as possible. Plug in devices also often consume power even then they are turned off, this is called phantom power consumption. The amount of phantom a plug in device varies from device to device but regardless it adds up over weeks, months, years.
You are correct in your thinking, you just didn’t put “lazy” product designers into consideration. 