TreeListBox SelectedItems is ReadOnly can't bind with MVVM structure.

TreeListBox SelectedItems is ReadOnly can't bind with MVVM structure.

I still made it possible to receive the SelectedItems in my ViewModel with the help of: https://stackoverflow.com/a/7227295/13164285

Meanwhile my question is why is it ReadOnly and how could it be better implemented in MVVM way without bypassing the whole MVVM structure like Fredrik Hedblad's solution.