Conditional Access expression cannot be assigned - C# null-propagation += events

One of my favorite C# features added is the "null-propagation" in CS6.

This has cleaned up so much code for many of us.

I came across a situation where this doesn't appear to be possible. I am not sure why as I though the null-propagation was just some compiler magic that does some null checks for us, allowing us to maintain cleaner code.

In the case of hooking into events..

 public override void OnApplyTemplate()
    {
        _eventStatus = base.GetTemplateChild(PART_EventStatus) as ContentControl;

        // This not permitted and will not compile
        _eventStatus?.IsMouseDirectlyOverChanged += EventStatusOnIsMouseDirectlyOverChanged;

        // but this will work
        if(_eventStatus != null) _eventStatus.IsMouseDirectlyOverChanged += EventStatusOnIsMouseDirectlyOverChanged;

        base.OnApplyTemplate();
    }

    private void EventStatusOnIsMouseDirectlyOverChanged(object sender, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
    {
        throw new NotImplementedException();
    }

Compile output shows:

 error CS0079: The event 'UIElement.IsMouseDirectlyOverChanged' can only appear on the left hand side of += or -=

So, my question is - what and I misunderstanding about null-propagation? Why is this not a permitted syntax?