Valve has updated its user agreement policy to make it against the rules to promote a product if you're receiving any commercial kickbacks without at least disclosing this.
"If you use Steam services (e.g. the Steam Curators' Lists or the Steam Broadcasting service) to promote or endorse a product, service or event in return for any kind of consideration from a third party (including non-monetary rewards such as free games), you must clearly indicate the source of such consideration to your audience," the new subscriber agreement stated (thanks Gamasutra).
Just last month Valve criticised the practice of gifting games to users in exchange for Steam Greenlight votes. "When you give away copies of your game in exchange for votes, you put us in a really uncomfortable position," said Valve's Alden Kroll a statement to developers.
Hopefully this new policy will alleviate such shady practices.