Because... the WoL could literally kill every single one of them then and there if he wanted to? If the point was to subjugate them, we would already be doing it lol. Also, we had no problem knocking out tempered Garleans on our way there. It would've made much more sense if, instead of letting ourselves be captured, we just knocked everyone out and left them with some supplies that they would wake up to. Then they could choose whether to eat the food or not, and they could find out after the fact (when they aren't subjugated) that we were honest when we said that we're just there to help and not take their country over.