I know this is an old thread but I have been struggling with this as well and I have a few machines that do this and a few that work properly and allow me to edit the interface. I found a way to get this working and wanted to share the solution here.
If you can't edit the default auto connections the edit the file /etc/NetworkManager/nm-system-settings.conf or /etc/NetworkManager/NetworkManager.conf (depends on the version you are using) and it should look something like this:
Add a line to tell Network manager not to auto configure interfaces (replace 00:00:00:00:00:00 with your mac address, multiples separated by commas)
now restart network manager
Then open the edit interface and add the wired connection manually and make sure you fill in the mac address field.
sudo service network-manager restart
*** Update ***
I just found out what is going on when I read this page from Network Manager
At the very bottom of the page it talks about Administration and Privilige and indicates that it uses PolicyKit for user rights and sure enough on the machines it didn't work on they didn't have policykit installed so a simply running the command below and rebooting fixed the issue without any other configuration.
sudo apt-get install policykit-1-gnome