IBOutlet for NSTextView in a ARC project

As you read here in most cases a IBOutlet should be weak.

Now as you can read in the development library not all classes support weak references. (e.g. NSTextView). This means you have to use assign:

@property (assign) IBOutlet NSTextView *textView;

If you use a weak reference you will get the following error: "Synthesis of a weak-unavailable property is disallowed because it requires synthesis of an ivar of the __weak object"

What the documentation missed to mention is now you have to set the property again to nil after it's usage e.g. by a dealloc method:

- (void)dealloc
{
    self.textView = nil;
} 

As far as I understood classes marked with NS_AUTOMATED_REFCOUNT_WEAK_UNAVAILABLE don't support weak references but what is the reason?