Draco knows Snape was a Death Eater with his father. He doesn't know Snape defected - Lucius has never told him for fear of awkward outbursts.

He certainly doesn't know why Snape joined up: knows nothing about the lonely teenage boy who felt rejected by everyone in his world except Lucius and his friends. All he knows is the embittered man that boy became.

And it's probably a good thing that he doesn't know about his father's methods of persuasion: the one thing no teenage boy, anywhere, could ever resist, especially if he believed it was love.

Lucius won't tell him. It might give him ideas.