Valve has asked developers to not give out Steam keys in exchange for Greenlight votes.
The message was sent to developers via an announcement page, which is locked, but a screencap of the message can be read on the Steam Database Twitter account.
“When you give away copies of your game in exchange for votes, you put us in a really uncomfortable position,” writes Valve. “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.”
Valve goes on to mention that giving away copies of a game in exchange for votes could set a bad precedent whereby other developers see what’s happening, and then think that’s what they need to do to ensure votes. It’s a system which Valve doesn’t think is “healthy” or “what customers want.”