[sbierwagen]

If Microsoft sends an email to Valve telling them to delete certain unlawful posts on Steam, or else they'll use Windows Defender to block the Steam executable, would you defend that?

Why not? It's not like Valve owns the computers of their customers. They're in clear violation of the Windows terms of service. If you violate the ToS, then Microsoft has every right to remove you from their operating system and Valve is in turn free to migrate to a different OS.