WPF why is my ScaleTransform Frozen and how can I bind to it?

Question

I have a pretty simple user control that I want to bind a ScaleTransform property to a DP in the code behind like so:

<UserControl 
x:Name="RoundByRound"
DataContext="{Binding RelativeSource={RelativeSource Self}}"
...
>

<Canvas x:Name="MyCanvas">
  <Canvas.RenderTransform>
    <TransformGroup>
      <ScaleTransform ScaleX="{Binding ZoomTransform.ScaleX, ElementName=RoundByRound}" 
                      ScaleY="{Binding ZoomTransform.ScaleY, ElementName=RoundByRound}"/>
      <SkewTransform/>
      <RotateTransform/>
      <TranslateTransform X="{Binding TranslateTransform.X, ElementName=RoundByRound}" 
                          Y="{Binding TranslateTransform.Y, ElementName=RoundByRound}"/>
     </TransformGroup>
   </Canvas.RenderTransform>
</Canvas>
</UserControl>

and then in the code behind I do this:

ZoomTransform.ScaleX = 3;
ZoomTransform.ScaleY = 3;

But I got an error saying:

Cannot set a property on object '...' because it is in a read-only state.

So I changed it to:

var cloned = ZoomTransform.Clone();
cloned.ScaleX = 3;
cloned.ScaleY = 3;
ZoomTransform = cloned;

But now that actually does nothing... no scale gets applied to my canvas.

HOWEVER

If I remove the binding on the ScaleTransform and just have it as an empty XAML element:

<ScaleTransform />

Then in my code I do this:

(MyCanvas.RenderTransform as TransformGroup).Children[0] = new ScaleTransform(3, 3);

It works! I get the scale applied

So 2 questions:

1. Why is my Transform Frozen is the first place?
2. Why doesnt my binding work when I clone the transform?

Thanks all!

UPDATE:

Here is the definition of the DP:

public static readonly DependencyProperty TranslateTransformProperty = DependencyProperty.Register("TranslateTransform",
            typeof(TranslateTransform),
            typeof(RoundByRoundControl),
            new PropertyMetadata(new TranslateTransform { X = 0, Y = 0 }));

        public static readonly DependencyProperty ZoomTransformProperty = DependencyProperty.Register("ZoomTransform",
            typeof(ScaleTransform),
            typeof(RoundByRoundControl),
            new PropertyMetadata(new ScaleTransform { ScaleX = 1, ScaleY = 1 }));

        public TranslateTransform TranslateTransform
        {
            get { return (TranslateTransform)GetValue(TranslateTransformProperty); }
            set { SetValue(TranslateTransformProperty, value); }
        }

        public ScaleTransform ZoomTransform
        {
            get { return (ScaleTransform)GetValue(ZoomTransformProperty); }
            set { SetValue(ZoomTransformProperty, value); }
        }