[newbie] Lack of security when booting in Linux single

[Jo]

Hi,

Recently my system failed to boot (I had added something to rc.local, that didn't 
belong there). On irc I was told that I could still boot if I gave linux single at the 
LILO boot. This worked, but to my surprise I never had to enter a login or a password. 
Even then, I was allowed to change rc.local back to what it was.

Is this normal? I thought Linux was supposed to be so secure.

Jo