Where on earth does this opinion come from? All sorts of vile stuff gets posted to both Reddit and Twitter, and they rarely-if-ever take any action against it. Certain subreddits will still moderate, but community moderators (users of some sort) generally control the moderation of those places, not Reddit or their staff directly.
And in cases where Twitter/Reddit directly take action, it certainly isn't based on an "MSM agenda", it mostly seems to only happen to limit their legal or financial liability.