I thought many moons ago someone mentioned on Pprune that AvData were acting illegally by intercepting the radio calls that enable them to charge. I haven't been able to find anything in the legislation but this dedicated site
SA Scan had this:
Contrary to widespread belief, scanning is perfectly legal. However, you should not use information obtained from listening for commercial gain or criminal activity. Monitoring phone conversations on your scanner is also forbidden. Apart from that, you can pretty well scan as you please, though discretion is advised when using scanners in public. For instance, being an 'ambulance chaser' and turning up at accident scenes, hindering emergency services is an extremely foolish and inappropriate thing to do. Genuine scanner enthusiasts don't want to give politicians an excuse to ban or restrict the hobby, as has occurred in other countries.
It's a bit like Optus picking up an AFL match and rebroadcasting to Smartphones, which is a case they recently lost in the High Court. It would be a valid argument that to intercept ANY data and rebroadcast it in any form (i.e. render and account to the the aircraft owner) would be the same as what Optus lost out on.
I wonder what would happen if you wrote to AVDATA and:
(1) claimed copyright to your radio transmissions from your aircraft and refused them permission to listen/record or re-transmit in any form,
(2) advise there would be a charge levied should they ignore your instructions.
(3) the levy for unauthorized use would be equal to the highest access charge on the AVDATA schedule.