Yes one could solve it with a good firewall setup also, but its likely the more complicated way and not all firewalls may be good enough to do it.
E.g. I havent found out yet how to allow access to a certain port only from specific IPs in my hardware router.
I am by the way right now using my world-wide accessible library as I am not at home where my NSLU is standing right now, but have a roku and a flatrate for internet also here so I was too tempted to just do it.
How hacker-safe that simple password system is I also dunno however. There I also had some doubts hence the feature request here. 😎