Valve has been cracking down on "games" designed to push achievements or Steam trading cards, and a new comment leaked from the Steamworks private group seems to indicate the company is looking closely at key requests as well.
A screen cap of a comment from Valve Software Engineer Sean Jenkin indicates that Valve is looking closely at developers asking for excess Steam keys, particularly if the requested number significantly exceeds the proportion of sales of the game on Steam. "If we are denying keys for normal size batches, it's likely your Steam sales don't reflect a need for as many keys as you are distributing, and you're probably asking for more keys because you're offering cheaper options off Steam and yet we are bearing the costs," he wrote (via Reddit). "So at some point we start deciding that the value you're bringing to Steam isn't worth the cost to us. For example, say you've sold a few thousand copies on Steam but have requested / activated 500K keys, then we are going to take a deeper look at your games, your sales, your costs, etc."
Keep in mind this is not an official policy (yet) and is just being discussed in the Steamworks group. However, the fact that Valve is doing extra monitoring of key requests is significant, especially if it affects legitimate indie game devs or publishers, or sites such as Humble Bundle. While it is likely that Valve's intentions are not to harm those groups, it is hopeful that the crackdown will continue to be on a case-by-case basis.
Valve has made a concerted effort to get the right games in front of its users, talking in the past about changing its Curator system and overhauling how recommendations are made. The site also ended its Greenlight program earlier this year in favor of Steam Direct to help streamline the publishing path.