Actually, Ed, it doesn't. It installs the plugin into the "Internet Plug-ins" folder in either
/Library/Internet Plug-ins
or the "Internet Plug-ins" folder inside your home directory
/Users/yourusername/Library/Internet Plug-ins
This way, any browser that is intelligent enough to use this plug-ins folder will be able to use Shockwave, and apparently this includes Netscape and Opera. Now if only OmniWeb would use it...