Let me try to break down what you're asking.
I find when use a jailbreak device I can access
Yes, that's correct. A jailbroken device can access an iOS device's root file system from a shell or any application (such as iFile). As of iOS 8, the Applications directory has changed to
But from this, I shouldn't?
Well, not necessarily. If the application is running as root (think iFile or Filza), you can have read and write access to any directory on the device.
Maybe I should write a App to scan the FileSystem on a normal iPhone and a jailbreak one.
In theory, any application can ask to read or write a file to any directory. This is frequently how jailbreak checks are done in production apps. If the file manager does not return an error when trying to read from
/var/mobile, then you know the application is being run as root on a jailbroken device.