I think koolyce was referring to the fact if you switch from defrost to any other vent position, the A/C indicator light turns ON.
There is no way to change this behavior. The feature is there so that if you switch from defrost to another vent position, the A/C continues to run to keep the windows from fogging. You could think of it as a safety feature!
marlin29311 is correct, the A/C runs all the time in defrost mode... even though the indicator light does not turn on.
Sidenote:
At least it's better than previous generation Nissan's... once the defrost was selected, the A/C would keep running regardless of the vent position! The only way to shut the A/C off was to turn off the fan OR stop the engine! That was a real PITA. You don't know how many times I stopped at a light, shut the car off and restarted to get the A/C to turn OFF!
BTW, there is a link in the FAQ that describes this 'behavior:
zer...stion