Vista Tutorial - WPF Lookless control that inherits ComboBox - IsEditable problem

I've built a "lookless" control that inherits ComboBox. It has three
dependancy properties (CountryCode, Label, and ListByName), and a ReadOnly
property of DistrictList.

Whenever CountryCode changes, the Label is changed, and the DistrictList is
rebuilt. (eg. Country changes from CAN to USA, Label will change from
Province to State, and the list contents will change from provinces to
states). When the ListByName property changes, the items in the list will
flip from names, to two character codes, or back again (eg. Ontario to ON, or
ON to Ontario).

In my Generic.xaml file, the ControlTemplate uses a dockpanel that contains
a label and a combobox. I've bound the label Content property to the control
Label property, and I've bound the combobox ItemSource to the control
DistrictList property.

So far so good... I can flip between CAN and USA, and the control flips
between provinces and states.

However, I also want to support a country of "Other", and when that country
is chosen, the DistrictList that gets built is empty. So after building the
list, I check its Count property. If it is 0, I set IsEditable to True, else
I set it to False. That way, when there are no items in the list, the user
can type something in, but when there are items in the list, the user can't
type something in and must select something.

Changing the ComboBox's editable state is the part I can't seem to get
working.

I have subs that fire when the CountryCode and ListByName depedancy
properties change. Inside those subs they call a private method called
LoadDistrictList. That is the method that fills the private member variable
for the DistrictList property, and that sets IsEditable to true or false.

I was ablet o get this working by creating a new dependency property called
MyIsEditable (and MyIsEditableProperty). I then added IsEditable="{Binding
Path=MyIsEditable,RelativeSource={RelativeSource TemplatedParent}}" to the
combobox in my Generic.xaml for my control. (I also did the same for
IsReadOnly.)

Could someone explain why I can't use the IsEditable (and IsReadOnly)
property on my control (inherited from ComboBox), directly?