Developers received the message via an announcement page, and Steam Database took a screenshot of the message before sharing it on their Twitter account.
Valve writes that the issue of giving away copies of games in exchange for votes has "been coming up more and more lately," so the company though it best to address it.
"When you give away copies of your game in exchange for votes, you put us in a really uncomfortable position. We do not think these votes accurately reflect customer interest and it makes our job harder in deciding which games customers would actually buy and play on Steam," wrote Valve.
Valve goes on to say they think that such practices are not "healthy for the [Greenlight] system" and aren't really "what customers want." The company thinks that with developers giving away the free games it could encourage other developers to do the same to get noticed.
Valve continued by saying, "We understand that running contests or giving away copies of your game can be viewed as a form of marketing. 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.”