Those using "Motion" are probably port-forwarding the webcam stream so they can see it outside of the LAN. The stream is not password protected, so your only security from others seeing the webcam is the port number, which isn't a very good security measure.
There is a patch to add authentication to the Motion webcam stream. But I don't understand how it work. This is the developer's wiki page for the patch. Maybe someone with more Linux knowledge than me can help.
http://www.lavrsen.dk/foswiki/bin/view/ ... mAuthPatch
As of the latest build, there is no "stream_auth_method" option in the motion.conf file, so this patch is not incorporated into the main install.