In Metro-style the sandboxing works that if something gives you a file (e.g. the picker, or a share operation), you can access it regardless of where on the system. If you find the file yourself, you have to declare the type.

The reason why I think it's related to sandboxing is because if you work with files programmatically you have to be explicit about the file types. This is to stop malware that you think is only interested in - say .PDF files, scanning and uploading any .EML files that it can find on the machine. It follows then on the pickers that restriction would continue. It allow's the retail store team to validate that an app is likely to behave itself. If it's an app that works with images, locking down the picker so that it can only access image file types makes sense.

However Matthias mentioned that he has an app that should allow files of any arbitrary file. That fits more into the "if the user selects it, it must be OK" camp than the "programmatic scanning" camp. So now I'm left wondering why the picker doesn't allow any type to be selected.

I think then maybe the decision comes down to simplicity. A lot of the decisions in Metro-style design relate to ideas about "zero intimidation". Allow the user to select any file is too much like Old Windows, and not enough like Reimagined Windows. What happens in Matthias's app if the user selects Explorer.exe as the file he or she wants to work with? I guess it's fine if you expect your user to know what they're doing (Old Windows), but not so fine if you're expecting a three year old to work with it (Reimagined Windows).