It really isn't an issue. Hamachi is designed to treat users from across the internet as if they were part of your local area network. Your buddies/creepy strangers that you connected to through Hamachi were treated as being on your LAN and hence were allowed to join your server, even if it was an internet one.
Just like my brother whom's computer is on my LAN can join my internet server, your Hamachi friends can too. However they cannot join other internet servers.