Because that was the only chance to get off the planet, so far as you knew at the time. I admit the explanation is a little weak, but there it is.
I don't think the Sand people quest is as simple as you say. I think to get the peaceful resolution you have to be disguised and get HK-47 to translate...
You can't enter the camp unless you wear the sand people robes, which you get the moment you step outside the port. So, you'll be disguised when you talk to them and it's made clear when you land that HK-47 is a translator so you better have him with you if you want to talk to local creatures.
I'm pretty sure you can enter the camp without disguise but you have to fight all the turrets. Also to get HK-47 is a little sub-quest in itself. I'll admit that the decision with the werewolfs has more emotional impact than deciding to kill the Sand people, but the point is it's something you work towards through the entire quest, instead of just being given a choice at the end. And you don't necessarily get dark side points, since they attack you on sight if you don't meet the pre-conditions and also even with the peaceful resolution you can insist that all attacks must stop, at which point they'll attack you.
Also there's other stuff to do on Tatooine, not just the Sand people quest.
Edit: Btw, in DAO you can't be selfish and evil at all, since if you were you'd just high tail it out of Ferelden, and let others take care of the problem, instead you have to go around saving everyone.