Sith do love gray areas, though. Palpatine was able to lure Anakin to the dark side by first bringing him to a gray area of morality. Claiming that to understand the force you need to understand both the light and dark is about as gray as it gets. The Jedi way is "Light side = good, dark side = bad," whereas Sith claim that morality is relative and that it's up to the individual to determine what is right and wrong. The reason the line is so controversial is because it's the straight up opposite of how the Jedi and Sith actually are, and thus has no place in the story. If anything, it should have been Anakin telling Obi-Wan that "only Jedi deal in absolutes. I'm finally free of their single-minded dogma!"