Oh that’s true in fairness i guess they would have to conduct a seance to ask the spirit of the person instead of raising them first and asking questions later.
In the novel ‘Before the Storm’ being raised as Forsaken is referred to as ‘The Gift’ and it’s in part down to how Sylvanas and the Val’kyr redefined what being raised as Forsaken meant. After her suicide on ICC - and the visions she received of Garrosh using the remaining Forsaken as frontline fodder - she returned with a way for them to have a future of substance as a true ‘race’.
The curse of the Forsaken was more than not being able to taste or procreate etc, the worst part for most was the rejection from the living; being treated like monsters by their own familes and never being able to return to places that were once home.
By providing an ostensible choice to the newly raised Sylvanas had ‘solved the plight of the Forsaken’. They weren’t doomed to slowly deteriorate - a fate the Lich King 's actions inflicted upon them - rather they could form a new undead future made up of only those who wished to be a part of it. They actually had some hope for the first time since becoming Forsaken.
I guess the problem is they are viewing ‘free will’ in the context of the Forsaken vs the Scourge. Like you say, raising them even to ask the question has already violated that persons free will but the fact they are given the choice Arthas never gave them has allowed the Forsaken propaganda machine to frame it differently.
I mean I personally have always found the Forsaken questionable (despite really liking the character of Sylvanas up until Legion) so I can’t say i agree with their reasoning but i think that’s supposed to be the crux of their post cata thinking. Undeath was framed as Gift, a second chance they were offering rather than the forced graveyard conscription it looks like to other races.
I had to stop myself the whole time trying to write any of this of using this line of reasoning but - its a game. The fact the forsaken have to remain a playable race really messes things ups since the likelihood of other races allowing them to continue doing this is pretty low in my opinion. Multiple leaders made crystal clear they didnt wanted Sylvanas raising their soldiers (see Lor’themar in SoO) so the fact they’ve continued to let her raise human corpses is really bizarre.
In reality i think a mix of both horde and alliance would have done everything in their power to kill her Val’kyr once they knew what she was doing and before she could raise an army for herself.