Only the admin user can edit stuff in defaults. There's a few things like that in concrete5. For example, only the admin account can log in as another user from the user details pane in dashboard.
We do this for permission reasons. Basically once you're in defaults, you can do anything. You can change stuff you may not have access to, even if you are in the administrator group (which really is just a group like anything else).
If you want to let your clients make changes across the site, we suggest using the scrapbook stuff in the dashboard, and then place those blocks in defaults. You can give them access to the scrapbooks to make content changes, and not the other stuff that getting into page defaults would allow.
For more information, visit Permissions.