Originally Posted by
War-God Matt
Hearthstone isn't exactly canon WoW lore. It draws inspiration from WoW, sure, but it is by no means canon.
For example; Hir'eek is not fit to be an evil warlock loa, they're using his G'huun-corrupt version while the non-corrupt Hir'eek is supposed to be a kind loa, according to his "Tales of the Loa" found in Nazmir.
"Hir'eek guards de weak,
He can find de things you seek.
He be blind, and so kind.
He brings clarity of mind.
His screech dat you hear,
Can dispel your every fear.
Softest wings glide through de night.
Taste and sound bring different sight.
When ya lost and all alone,
In de darkness where you roam,
His voice will guide ya home."
Hearthstone devs just take things they find cool no matter if it breaks the canon from WoW, which is fine. It's a different game.
Anyway, I am in support of Zandalari warlocks, they're the oldest sentient race on Azeroth, they're scholars that value knowledge, most of them live to protect their home. It makes sense that they strive to master and control most if not all types of magic just so that they can protect their home and loved ones. And master magic they have, they're incredible and fast learners (as said in Shadows of the Horde) and on the Isle of Thunder, the Kirin Tor and Sunreaver Onslaught mages noted how the Zandalari mages' mastery over the arcane and shadow was impressive to the point where they combined the two schools, something very few had accomplished on Azeroth.
As such, having Zandalari who mastered the usage of fel and demons to be able to defend their homes better from demons and other fel-users (and of course just better in general) makes 11/10 sense and abides to the lore about Zandalari morals and their scholarly nature.
And I very much doubt that the Zandalari haven't ran into the Burning Legion at some point during their several invasions.
So rather than trying to justify it using non-canon Hearthstone stuff, go by the fact above, that Zandalari are scholars that seek knowledge, something that has been canon lore since vanilla, through WotLK and even in BfA.