This person never played the Warcraft III Orc campaign or The Frozen Throne's bonus campaign. As you can see in Durotar to this day, the Alliance actually DID invade the Horde's new home without any provocation, only to be defeated by Rexxar, the forces of Orgrimmar, and Theramore loyalists. Thrall is also one of the most compassionate and forgiving characters in all of Warcraft.

The arenas are technically owned by the goblins, so the orcs have no real sway in the politics of it, mainly using it as entertainment and business. Rehgar was raised a gladiator, and now he trains gladiators. He also lets them go willingly, he is not an evil person. He saw Varian as a "Theramore deserter", orcs believe strongly in honor, and deserting your people is very dishonorable in their eyes. Varian was seen as an untrustworthy human because of this, and was only then taken to be a gladiator.

"At Dire Maul, I recouped my investment in those two a thousand times over. I knew this day was coming. A man is truly a prisoner only as long as he agrees to remain one. After that, in his heart, at least, he is free. And where his heart is, his body may follow if his will is strong enough." -Rehgar