The keyword is "usually". Those are the choice of the interrogator, and not something they absolutely *have* to do no matter what. Funny that you mentioned the Inquisition, the witch hunts and such - surely you are aware that people consider those messed up, yes? Not to mention that in WoW, there are spells that can read people's mind in WoW - one that even a Dalaran apprentice is aware of - I have a hard time believe that Sylvanas wouldn't be able to find any magic caster in the Horde being incapable of using it. Being cruel in this case is a choice, not a must. If you are trying to say that Sylvanas, and maybe some of the Forsaken (not all of them, obviously, as the Gathering has shown that there are - or were - a number of Forsaken capable of positive emotions) is cruel and cold-hearted then yes, she is / they are. That's the point. However, make no misunderstanding. There are other viable and way more benevolent options that could have ended much better for those Forsaken who were returning to her side. Sylvanas just choose not to pick any of those.
Assume we flip the situation around, and a group of Alliance members who *might* (not a confirmed fact) want to leave to live with their Horde friends. Can you imagine Anduin shooting them all down, no question asked?