Every article that has a bold, red "M" before the article requires premium membership to read - take note, for instance of this page:
These premium articles are world-class quality articles. The founder(s) of this web site, some of them, have PhD degrees in Engineering, etc., and they do very methodological quality/quantitative analyses of topics of extreme interest to the sport. By becoming a member, you are supporting the staff, the site, these writers, etc.
This is such a great web site, like a homebase away from home. If you enjoy the site's free features, you'll really, really enjoy the site's premium features and you'll feel good supporting these endeavors and people.
I don't work for BPL, I'm just a big enthusiast of the site. I call BPL the "Science magazine of Backpacking".