So, books, magazines, and virtually every form of print should have margins, but the web should have the text go right up to the edge of the browser?

I disagree that a sensible max-width is wasted screen real-estate. Even after resizing my browser I'd prefer my content to have a little bit of space to breathe.

I just realized that the comment section of hackernews uses ~80% of the window width, with hardly any padding for top level comments.

100% of the window is wasted if the lines are too long to read.