If they use USD on an australian site (thinking from a business owners perspective) it could be to keep the prices true for them. So for example an MP will be $50 USD and given the fall of the AUD against the USD it would protect the business against possible loss.

The COST is the same for the business and the PRICE is whatever the conversion ends up being on the day for the customer.

Does that make sense? (any economic heads out there?)