I'm not sure where you see a contradiction. When a user downloads and uses the Lion installer, as the installation completes, the installer self-deletes from the user's Mac.
Completely, separate from that, all apps that a user has purchased can be redownloaded from the Purchases pane of the Mac App Store, as long as the app is available from the MAS. Because the Lion installer is close to a 4 GB download, it is good to tell folks that they might want to make a copy so they don't have to download such a large file multiple times, especially if their ISP limits their bandwidth.