Air conditioning stay on on defrost

The round switch you turn to make the air come out the dash vents, floor or defrost. If you have the switch in any of the last two settings "defrost"modes that by-passes the push in button switch. Because any time it is in defrost mode the A/C compressor is engaged. Remember the old cars pre-1980 when you turned on the defrost the windshield fogged up that was because it had moisture from the heater in it. Todays cars kick the A/C on because they want to to mix the warm air with the A/C air to make dry air to clear the window quicker.
Hope this helps