You are correct. Currently, when you right click in RadListBox, this changes its selection. We will change this behavior and will follow the Microsoft approach - in their listbox you can select items only with the left mouse button.
Until we make this modification, I have implemented a workaround for you which will work just as you expect. With your left mouse button you will select items and with your right mouse button you will open a RadContextMenu without changing the selection.
I have updated your Telerik points for bringing this to our attention. If you have additional questions, feel free to contact me.