I clicked on the link (after posting the question myself!) for info, and got the following:
"In our forums, you'll see contextual links to purchase gear from third parties. These links are served by GearBuyer.com, and when you purchase gear via these links, we receive commissions on the sale. We do our best to develop algorithms that help you match forum discussions with shopping links that are related to those discussions, and to help you find the gear at the best prices.
Because traffic to the forums, which remains a public service by Backpacking Light, requires significant amounts of bandwidth, buying bandwidth represents our single biggest expense at BPL. So when you buy gear, that really helps us keep the forums public, and free.
Thank you for supporting BPL!"