Why does it have a monthly fee? What are the differences between the US and AUS markets that require Australian users to pay three different fees to use a payment gateway?

I was part of the beta and wondered that too. It's almost like they don't expect to cover their internal costs with their (reasonably pricy) cut of each transaction.

From what I've heard, the Big Four banks aren't being the most cooperative, so my guess would be that Pin has had to make concessions to them.