WordPress like most CMS websites has 2 types of users - unknown users and authenticated (logged in users). In fact pretty much every website or analytics systems tracks users based on this user profile.
Unknown users can only browse the front end and have no access to wp-admin. This is to be expected, otherwise explain why it is it that an unknown, not logged in user needs access to wp-admin and go raise a feature request on wp-trac.
Now ... you could create a Guest account with a password like 'demo' but you want the user to have browse only access to 'all' of the admin section.
Again, this is not possible, and it is the standard for all CMS solutions from WordPress to Drupal. Authenticated users must have either read-only or read/edit user privileges to a system function.
Core Requirement of Web Content Management Systems
What should be simple to avoid for you, is actually a core feature to all web CMS solutions - that is ...
- site visitors are either browsing or logged in; and
- logged-in users have role privileges that limit their access