I take it you haven't done Ashenvale.
Basically, no, the Horde is not more pleasant to warlocks, at all.
In no way is Garrosh more tolerant of Warlocks than Thrall - I'd check up on your facts. If anything, he's less accepting of many, especially warlocks. Thrall used warlocks to track the Burning Blade.
Garrosh kills warlocks.
Garrosh also puts a bounty on your head for working with demons.
The Forsaken were backstabbed by Varimathras, and demons MADE the plague, the very reason they were enslaved, something they absolutely /loathe/.
The Orcs were corrupted by the Burning Legion.
The Blood Elves were nearly ruined by their involvement with the Legion.
Tauren have /always/ been wary of unnatural magic, and until recently didn't even have a warlock trainer in their city.
They don't /thrive/ in the Horde, far from it - if anything, the Horde is just as wary of the Alliance, with possibly even more reason to be. Look at how many races it's effected within the Horde, on a massive, personal level.
Also, it wasn't an Orc who summoned an Eredar in an Argent tournament, if I recall correctly.
As for the Goblin/Troll trainers, they added them for game mechanics, I'm fairly certain. Orgrimmar is much smaller in game than it is in lore, and these races needed a trainer too, for mechanic purposes. If there was an underground for those races, you can be damn sure they'd be there, especially in Hellscream's Orgrimmar. Non-Orcs/Tauren are actually banned from a good part of Orgrimmar, as well, as far as the lore is concerned, so they're not in places Orcs would see them, either.
Also, anyone with any class, lore-wise, avoids the slums.
Edited by Vanyris on 5/13/2012 3:16 PM PDT