Good question, and I think there are a few ways to approach this. My first answer would be to note that the author more or less lays out their thesis at the end of the first paragraph. E is a pretty good paraphrase of that least sentence.
More importantly we should eliminate B and C, which look pretty strong directionally. I think the author would agree with B, but is it the main point? I think this might only apply to the driving example, not to the steroid example. B might be a good summation of the second paragraph, but I don't think it captures the argument of the passage as a whole.
I think that C might actually slightly misconstrue the argument. The author is saying that laws that enforce conformity can be valid because that are actually reducing harm, and conformity is merely a means to and end. C seems to be treating conformity as the end itself, and harm reduction as a justification for that end.
This is a tough question and usually for main points we can eliminate wrong answer choices, and come to the right one by process of elimination. In this case however I was only confident that B and C were incorrect after I saw how closely E tracked the main point the author set out at the last sentence of the first paragraph.
