RelayCommands overriding the "IsEnabled" of my buttons.

Is this is a bug? Here is xaml from my View and code from my ViewModel

<Button Grid.Column="0" Content="Clear" IsEnabled="False" cmd:ButtonBaseExtensions.Command="{Binding ClearCommand}"  />    

public RelayCommand ClearCommand
    {
       get { return new RelayCommand(() => MessageBox.Show("Clear Command")); }
    }

Notice I hardcoded the IsEnabled="False" in my xaml. This value is completely ignored (button always enabled).

I realize that RelayCommand have a CanExecute overload but I did want to use this as I want to do more than just have a disabled button.