@Bapo
They are given a choice, once they CHOOSE to follow Sylvanas, their loyalty to her is mandatory. The act of treason is most places, real and fictional is cause for death!

Take Koltira as an example, he choose to follow Sylvanas and the did something that could be considered treason to the Forsaken and the Horde, and he was/is being punnished.

Others choose not to be part of the Forsaken, look at lilian Voss as an example. she chose not to be part of the Forsaken, but in place of that chose to go on a murdering spree against those who would not help her. She was not forced into servitude, and she was allowed to go about what she wanted to do, or needed to do. A powerful ally she would have been for the Forsaken.

I think the choice of the Forsaken is not in question at all. I think the bigger issue is the Forsaken going against the horde several times, and Garrosh will need to remove the burden of Sylvanas' head from her shoulders...Mind you when that day comes, I will be in the corner of Sylvanas.