Is trying to derive a person’s preferences from tangentially-related data immoral, or does it only become immoral when it’s done in an attempt to sell a product?
Deriving preferences is not inherently bad. It's immoral to undermine people's will for profit. This is the same reason the gambling industry is immoral, and making manipulative free-to-play mobile games is immoral. You're basically hacking people's subconscious for your own gain at their loss.