Missy they don't. It is the a/c flying into or around the controlled aerodromes (and other aerodromes) that can cause the trouble. The VFR pilot can (and do) leave the farm or a CTAF/MBZ with it switched off. We will never know where they are. Contrary to Dick Smiths statements that you will only have a mid air near the departure/arrival areas of aerodromes, the recent incident near Mel clearly shows it can happen anywhere. VFR a/c flying along at 17500 with their transponder switched off is nothing other than a disaster waiting to happen. And because the services have been reduced to somehow enhance safety, yet another layer of the safety net is removed.
How on earth you teach someone situational awareness is difficult to tell these days. As Mr Smith says, once you leave the departure airport, you dont need to listen to anyone until you get close to the dep/arr area of the arrival airport. What a neolithic concept! Neanderthals, all of them!!!!