They aren't sold off as such, more they (the govt) let private operators run the service, ie, a concession.
They let the private operators run, in some cases, the govt sets the fares, still, and let the private operator do the operation, and then pay the govt the revenue owed.
PTV is still a Vic govt owned agency, but then the individual services are run by the private sector, the trains are still owned by the govt, they still have their fingers in the pie, the SA govt runs AdelaideMetro, but the bus operators are private, the Sydney trains are still govt owned, but operated "privately", Sydney buses are govt owned but run by John Holland, our airports are still Aust fed govt owned, but allowed to be managed by private businesses for profit.
Skybus in this equation, is private, yes, but they would still have to pay the Vic govt something, pay the airport authorities something, pay the toll operator something, overall maybe Skybus keeps, I don't know maybe $10 per $18 trip.
Looks like the Skybus app does not in itself provide any rewards, <(Off T) but at the least, I can safely keep my Skybus BP from being lost, unlike my last VAd trip BP, which came out of email as a meaningless list of alphabets, but no VAd BP (On T)>.
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With Adelaide bus drivers now not accepting cash, and they usually let a lot of free loaders on, it was their union rules not to push to collect fares, a lot of the back door entry of pax onto Adelaide buses are free loaders.
So, from Adelaide Airport, or on any buses, there are ways to get around gratis, not that I am saying its the right thing to do, but with not a lot of "Prescribed Officers/Customer Support" ticket checkers, its all on for free.
Note: I am not saying free loading is the right thing to do.