that's not the only protection in that zone.
if they try that they are isolated from the entire network until the username/pass is entered for the network.
point is. if you aren't in the whitest you can connect to a low quality Internet access an file box for sharing movies and music. but anything "personal" is on a seperate network.
a bit like the cable companies do on their routers. only with file sharing.
lots of ways to do it. but my point is people should already be running at least two networks anyway. one for them and their devices and one for guests.