Airport Extreme multiple SSID password fail?

So I recently went over to a friend's house and connected to his wifi (Airport Extreme). He has it "partitioned" into 2 different SSID's. The main called : Deep Space. and the second called : Ground

Ground is used as a guest network.

Both SSID's have wpa passwords.

So I connected to ground and then when I tried to connect to deep space manually using the menubar it logged me straight in and gave me an IP and everything

I didn't even have to put in the other "network"' password.

So on an Airport Extreme, if you have multiple SSID's and log into one of them, should I be able to switch to another SSID I have never logged into before, with a different password without typing in the password?

Seems like a security flaw of the Airport Extreme to me..

Help?