>I think the issue is - who gets to decide how wide the content is. Is it the user or the website? Or to put it another way, does the act of maximizing a window represent an intent or does it represent a default error state that needs to be worked around by laying out the content in a different way.

This is why CSS exists - it separates style from content - if you want what that site has just disable all CSS and you'll get raw content.

>What are those non-subjective reasons?

There are things like optimal line length based on eye travel distance and a bunch of "defaults" that have been found trough centuries of experience in design/print. I don't do design professionally so I don't remember the exact rules but I did take design classes back in high school so I know they exist - I'll let someone who's an expert fill in on what they are :D