Valve has asked Greenlight game developers to stop giving away game keys.
The company isn't taking action to stop this practice (yet), but it posted an announcement on Steam Developer Community conveying its quibble and hinting at possible rules change.
Valve conceded that giving away free game copies can be an acceptable form of marketing in a lot of cases, but not on Greenlight which needs to gauge customers' interest in each game.
"We understand that running contests or giving away copies of your game can be viewed as a form of marketing," noted Valve. "But for the purposes of Greenlight, we don't think that giving away copies of your game in exchange for votes accurately reflects genuine customer interest."
The full announcement is accessible only by registered Steam developers, but a screenshot is available in the gallery. (Screenshot is provided by Steam DB.)