OK last try from me and then I'll get off my high horse:
Amex is making a good faith offer to prospective customers. The obvious intent is that anyone taking up the offer stays on board for an extended period in order for the relationship to be profitable for both sides. Obviously there is no contractual way for Amex to enforce this so they have to trust that most people understand that commercial relationships are a two-way street, while also pricing for the small number of people who take advantage. (This is why churning is, in fact, taking away from other customers).
If you churn relentlessly, you KNOW you are going against the intent of the offer. Of course it is 100% legal, but my point is that if eveyone behaved like this there would be no sign up offers at all.
For me, a good commercial relationship creates value for both sides. If you play so hardball that you take all the value (and then some) for yourself, I think not only is that bad long term business, it is selfish the point of acting like a d*ck.
Now back to the topic at hand: if you get the letter saying it was due to something in your bureau, it means you are identified as a churner (or "gamer" as they call it internally) and they have every right to decline to do business with you. Going to AFCA? That's not playing the game hard, either, it is taking advantage of what is supposed to be a watchdog for legal breaches but using it as leverage, not a remedy.