This is by design. From the Using Sitecore Fast Query documentation at: http://sdn.sitecore.net/upload/sdn5/developer/using%20sitecore%20fast%20query/using%20sitecore%20fast%20query.pdf
Sitecore Fast Query can be used:
- In the Content Editor as a source for List Type fields.
- In the Developer Center, in the XPath Builder tab.
- In custom .NET code by using the SelectSingleItem() method and the SelectItems() methods of the Sitecore.Data.Database class.