As for why warlocks can summon them, while not demons Void-born entities can still be summoned to the material plane and bound with sorcery, so it's kind've a 'same general principal' thing.

The Legion on the Broken Shore was actually messing around with Void energies are trying to bind Void entities, including a 'Shackled Void Lord' of all things, so just like warlocks the Legion is going for a 'fighting fire with fire' thing. I'm sure that'll be fine.

Please Help.
Thanks !

I didn't find the right solution from the internet.
References:
-http://www.mmo-champion.com/threads/2247745-Are-the-Voidwalkers-and-all-the-void-creatures-still-considered-quot-Demons-quot
- Investment Management Video Examples