The Copyright Act does not expressly impose liability for contributory infringement. According to the U.S. Supreme Court, the "absence of such express language in the copyright statute does not preclude the imposition of liability for copyright infringements on certain parties who have not themselves engaged in the infringing activity.
One who knowingly induces, causes or materially contributes to copyright infringement, by another but who has not committed or participated in the infringing acts themselves, may be held liable as a contributory infringer if they had knowledge, or reason to know, of the infringement. See, e.g., Metro-Goldwyn-Mayer Studios Inc. v. Grokster, Ltd., 545 U.S. 913 (2005); Sony Corp. v. Universal City Studios, Inc., 464 U.S. 417 (1984).
IANAL. Considering Valve not only gives games a retail platform, has to approve games before sale, and takes a cut of that sale and assuming the reddit post isn't a lie then I am gonna guess Valve's probably well staffed legal dept decided not to take a seemingly iffy legal gamble on a game that probably wasn't going to rake in a ton of sales anyway.