To elaborate on Deathrey’s comments…
UE4 is doing this correctly only to ambient lighting, like your image says “AO should only darken areas commonly refereed to as 'ambient lit”. This happens after the ambient lighting pass (skylights/baked lighting) and before the direct lighting pass. This is efficient and no artistic choice is involved.
Unity examples above happen after ALL lighting. Adding a tuner for masking out AO based on direct lighting is not efficient, depending on the engine may require additional space in the G-buffer.
In ue4 If your direct light is dim, it will not overpower the ambient, and your ambient shadow (AO) will be easily visible. You can absolutely see this effect in real life on an overcast day where the sun peaks through the clouds, or during sunset.