Here's how I think of it: the kinds of magic out in the World of Warcraft are basically the same for any caster, but how you get to it matters greatly. Consider: there are no draenei warlocks.* Warlock magic uses fire and shadow. However, there are draenei shaman and mages who use fire magic, and there are priests who use shadow magic. The injunction against warlocks is thus not the kinds of power, but how they reach it - through demonic bargaining. What a mage does and what a warlock does are very different, even if they get the same fire out of it in the end.

Likewise, I see druids as hitting someone with the same "stuff" when they starfire as a blue dragon would, or a mage, or the arcane dot that nelf priests used to get back in the days of race-specific spells, and so forth. However, how a druid does it matters significantly. To that end, the arcane folks probably stole the principles from the druids. Some might disagree with this, but it's my interpretation of Warcraft magical cosmology.

It also points out a problem of language. "The Arcane" doesn't always mean the same arcane. For the night elves, "The Arcane" seems to largely refer to a method of acquiring power and the philosophy of it's users as much as anything, probably encompassing both mages and warlocks. To that end, a lot of it is probably cultural, where "Arcane" means "like the Quel'dorei do it" as much as anything else, and the Kal'dorei just don't like them that much. Likewise, orcs and trolls and tauren who refer to "The Spirits" can mean slightly different things in different contexts - ancestors, loa, or elementals.

*Strictly speaking, there are draenei warlocks. They're called Eredar.