The User Account Control doesn't perform checks on the safety of the software, but rather permission for elevation of authentication. Some apps will require entries to be created/modified in your registry as well as system folders. The user account that you're logged into does not necessarily have the access to write to these locations. However, you are presented with the option of continuing the installation/execution of the software through the elevation of privileges with User Access Control.
So it doesn't really have anything to do with the Metro apps or the security checks that it passes. Personally, I like knowing when something will be writing/accessing to system files and registry keys.