It also has persistent permissions.
Think about it from a real world perspective.
I knock on your door. You invite me to sit with you in your living room. I can't easily sneak into your bed room. Further, your temporary access ends as soon as you exit my house.
The same should happen with apps.
When I run 'notepad dir1/file1.txt', the package should not sneakily be able to access dir2. Further, as soon as I exit the process, the permission to access dir1 should end as well.
A better example would be requiring the mailman to obtain written permission to step on your property every day. Convenience trumps maximal security for most people.
> When I run 'notepad dir1/file1.txt', the package should not sneakily be able to access dir2.
What happens if the user presses ^O, expecting a file open dialog that could navigate to other directories? Would the dialog be somehow integrated to the OS and run with higher permissions, and then notepad is given permissions to the other directory that the user selects?