It's very simple, kaz.
If you call ATC to request a RIS or flight following, it's safe.
However, if you make a taxi broadcast of exactly the same duration on the same frequency, because you happen to be the one out of a million movements that isn't in the vicinity of an aerodrome depicted on aeronautical charts, it may rain aluminium confetti.
All the VFRs can request a RIS or flight following, and ATC has to respond, even if to say "too busy". That's safe, because the risk of the VFR requests over-transmitting safety-critical instructions to aircraft under air traffic control is dealt with by read-back requirements.
However, if ABC makes a broadcast, on area, taxiing at a property 20 miles to the southwest of Mildura, for Bullamanka to the west at 8,500', some safety-critical air traffic control instruction may be over-transmitted without anyone knowing.
Like CASA issuing corporate credit cards with the paywave function enabled, and CASA staff holding up their wallets to the paywave terminal with the expectation that it will charge a different card, it all makes perfect sense.
Last edited by Creampuff; 1st Nov 2014 at 03:32.