Classes generated from XAML (in WPF) are public by default

In Windows Presentation Foundation, classes generated from XAML markup have public visibility by default. Why not internal (like interfaces)? I don't want my application GUI private classes to be visible from the outside of its assembly. More over, it is not possible to alter that default in language-neutral way. One need to use x:ClassModifier attribute in XAML root element to change class visibility specifying language-dependent value (internal for C#, Friend for VB).

Links on the topic:
- Nice post about that problem
- MSDN article on x:ClassModifier
- Microsoft Connect ticket on that problem