Why wouldn't they when the original movie novels (PT + ST) are considered canon?
https://twitter.com/delreystarwars/status/461579307341840384
I think this is where the disconnect is. When Luke surrendered to Vader and the emperor, you are right, he didn't have the intention of killing Vader. He wanted to turn him to the light. However, as the fight progressed, he gave in more and more to the dark side. Eventually he did fight with the intent to kill. That intent was there all the way to the point he disarms (literally) Vader:
Remember Yoda's teachings? Where he says that you are in the good side when you are "calm and at peace"? He definitely wasn't calm or at peace there. Yoda also says a Jedi uses the Force for knowledge and defense, never to attack. What did Luke do there?
Now whether or not you count the novels, you can't deny the 1983 novel was taken from the original script. From the book:
Watch the clip above, it matches exactly what was written in the book. The book's publisher confirms that the books are canon where the films align. This definitely aligns IMO.
This is technically incorrect. Luke did not go in there with his lightsaber in hand. He went in there with his lightsaber hanging on his belt, like almost every Jedi (remember how Jedi aren't supposed to simply leave their lightsabers around?
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There's the scene in question above. He goes in, inquisitive. He is trying to read his student. What he sees terrifies him to the point where he ignites his saber. Look at his face, that's fear in his eyes, not murderous intent. He never even tries to swing his saber. The only move he makes with his saber is to block Ben's attack.