either way i do believe that the power for this field is very weak, otherwise (or it may be the case as well) samsungs engineers would have built some switch (soft- or hardware (sensor if pen is out)).
but generally high resonance surface fields (as it would be the one here) wont use much power anyway, so don't worry! it's more like a sensor array which detects EM "disturbances" created by the coil of the pen. pressing the buttons on the pen (eraser also possible and working on the note, but not with stock stylus) just alters this disturbance by shortening some wires of the coil and therefore generate a characteristic "fingerprint".
Disabling pen detection when the pen is attached can extend battery life