As Ziggy pointed out, the darkness should be related to the planet's location in the galaxy and the related star desity.
And as Ziljan confirmed,
this is already the case.
I do see the point, that the dark side could in fact
be a little bit darker in "our" (the human bubble) region of space.
However, the usual expectation of pitch black nights on planet surfaces comes (obviously) from our own experiences here on earth. Nights here can indeed be pitch black - but
only, if the night-sky is overcast and there is no moon and no stars visible at all. Clouds are an atmospheric effect, though and planets we can land on in ED don't have one.
A night with clear sky, on the other hand, can be really bright even here on earth, especially in regions with little pollution. I remember one holiday trip, where we camped in the middle of the Sinai desert (one of the best holidays in my life!

). The moon wasn't there, but the light of the stars alone made it
very possible to see and orientate! A newspaper probably would have been not possible to read, but a child's book with nice big letters would. And this is on a planet where the atmosphere (clear, but still present) did still filter some of the light.
Now imagine an airless planet!
Yes, there should be complete darkness in ED.
But applying this demand to each and every planet's night side would not only harm our ability to play the game, but also wouldn't be physically correct!