Is:
When Post Process Quality settings are on High or better, ambient occlusion is automatically turned on. One can turn it off again afterwards.
Upon restarting the game, it's on again.
Additionally, the state is not visually reflected in the GUI when you turn AO off, then set PP-Quality to High (or better).

Should(?):
The AO Setting should not be bound to the PP Quality setting, it should, as represented in the menu, be a setting on its own.
Additionally, it should probably automatically switch off, when post-processing is turned off, as it's a post process effect.