Because you’re twisting it. Let’s go step by step.
The Orcs committed numerous atrocities on their own world, and on Azeroth. They committed genocide against the Draenei, Arrakoa, and presumably the Saberon considering they’re nowhere to be found in Outland. They also killed a significant amount of Ogres and enslaved others. They built a highway out of the Draenei’s bones, and corrupted their own world and killed it. Then they invaded Azeroth, destroyed Stormwind, raided Khaz Modan, trapped the Dwarves in Ironforge, defiled human corpses by putting Orc souls in them and creating gen 1 Death Knights, enslaved the Red Dragonflight, burned down part of Quel’thalas, and more.
When they were defeated, 3 obvious options lay before the Alliance. Kill them (genocide), let them go (suicide), or IMPRISON THEM. Let me say that again. IMPRISON THEM. Notice how IMPRISON and SLAVERY are two different words. Crazy I know but true. And the original intent, was to eventually reform them, let them go, and possibly even have them join the Alliance.
The vast majority then became lethargic and useless without their demon crack. As in they would make awful slaves. I love the part where we go back to Durnhold Keep in Caverns of Time and they’re all being whipped and forced to do hard slave labor…oh wait…NOPE they’re all sitting around doing nothing.
The original overseer of the camps, Danath, had to leave for some random reason. I wonder what it was, I’m sure it didn’t have anything to do with Orcs stealing artifacts, working with Deathwing, and blowing up their own planet. So as a result, Blackmoore was put in charge, who was a corrupt piece of garbage.
This corrupt piece of garbage found an Orc in the wild, named Thrall and raised him to be a gladiator who fought other lord’s champions. Usually human criminals or animals, but there was one Ogre. But the horde doesn’t seem to care too much about enslaving Ogres huh? Yes, this one (two if you count the ogre) instance is very bad, and makes him a very bad man. Still following me here?
Later he made plans to make Thrall a General of the Orcs loyal to him, and USE THEM TO ATTACK THE ALLIANCE. You know, the faction you accuse of enslaving them? You guys love leaving that part out of the story huh?
So we have one corrupt piece of garbage, who wasn’t even the original overseer of the camps, which the original intention of said camps was to reform and release, who found and enslaved one orc kid and forced him to be a gladiator, not the other orcs already in the camps, and planned to use him to overthrow the Alliance using an Orc army.
And this is the grand history of Alliance slavery. Amazing. I’ll get to writing an Alliance version of the Emancipation Proclamation right away. Maybe we can finally free that Ogre, and then the Horde can re-enslave it.