Thrall claims, and takes pride in the fact that the modern Horde he envisioned would be about honor and redemption. He allows the Forsaken in however, which seriously makes you question his logic. Sure the origins of Sylvanas and her followers are tragic, but just look at what they do. They torture people, force them to labor, steal lands through threats of agonizing deaths, and produce some of the worst weapons of mass destructions seen in Warcraft.
It was Thrall himself who approved of their inclusion, not Garrosh. Not only this, he has been to Undercity, and until the Wrathgate incident, he still did not mind, and even gave Sylvanas back leadership, without criticism of the horrors he witnessed. The worst he did, was place Kor'kron in UC, but even then, this hardly deterred them from their usual activities.
Sylvanas to date, does not feel remorse for her actions, and discreetly uses the New Plague to command power, and commit atrocities (Southshore).
This is just ironic, because the Horde claims they preserve honor above all else, but the Forsaken are mostly allowed to do as they please, and in many cases, you help them.