When new video game stores were opening that charged much lower commissions than Valve, I decided that I would provide my game “Overgrowth” at a lower price to take advantage of the lower commission rates. I intended to write a blog post about the results. But when I asked Valve about this plan, they replied that they would remove Overgrowth from Steam if I allowed it to be sold at a lower price anywhere, even from my own website without Steam keys and without Steam’s DRM.

From the source cited by the article.