It's easy for you to quit smoking? Good for you. But it's very clearly not the case for most people. Feels to me a bit like saying "it's easy to be rich, you just have to be born in a rich family like me".
This is also called "executive functioning" and it's wholly dependent on your brain chemistry, i.e. the thing that is malfunctioning in addiction.
So yes, they lack "discipline" because discipline is also physiological, it's not just (and maybe not even primarily) a reflection of the person's character. By treating them as separate you're engaging in mind-body dualism.
How is it not black and white? You say "it's easy for everyone", I say "you're wrong".
If I'm right, you're wrong. If you're right, I'm wrong. It can't get more black and white than that.