WebMatrix needs an active internet connection to install since it pulls in some other required services from the Web Platform Installer.
Quote:
"..WebMatrix needs other components installed to function correctly, and those are all pulled in by Web Platform Installer automatically. In that sense, the true installer is not really "offline"."
Source: http://connect.microsoft.com/VisualStudio/feedback/details/641843/webmatirx-offline-installer