Yeah, I'll fix that. The problem is not that tar.bz2 files don't work, but that this archive has all files set to read-only. BetterZip first extracts to a temporary folder and then moves the files into the final destination. Now, moving read-only files needs extra authentication and BetterZip has no authentication mechanism yet. Try to delete the extracted QuickSilver.app after extracting with the Archive Utility. Finder will ask you for the admin password.