I don´t get it. I just tried your source here and all seems well.
Check the image I attached and see if that´s "selected".
I´m a "PureAS3" guy so I don´t know this mxml and components stuff very well.
Testing again I noticed that I can select any combination in the list, but can´t deselect them. I don´t know if that´s the intention, but it´s what´s happening here.