Ubuntu is not Linux.. it's one seporate OS and user apps wrapped around a Linux kernel. Each distribution is different with different goals so I wouldn't mistake Ubuntu for being representative of all other OS that happen to be based on the Linux kernel. There was also a lot of discussion over the sudo setup when Ubuntu was first released so it's not like "Linux didn't learn from what they said". Actually, reducing default security is the very reason many techies stick with there own prefered distrobution rather than switch to Ubuntu or they aproach it like Windows and do the hardening themselves.

I just wanted to point that out before this degrades into an "Ubuntu reduced security so all Linux based OS must have reduced security."

I think most people agree with your second point; security by default. An admin password should be setup during installation just like the user account and password is setup. Cononical, as is there right, chose to build there distribution this way so the community responded by posting howto information on correcting that security flaw.