Because then he can post on here and other people can see that it's not possible to be hired if you aren't from Australia and don't have green card/passport, and won't be wasting their time.
I have seen nothing official from the company on who they hire. I have heard multiple times from new hires, or potential new hires that they were told something different from what you are saying.

Note: I just looked at the official Atlas Air qualifications for pilots. They require a passport with the ability to travel worldwide, unrestricted. That is all. No mention of US passport, green card, visa...nothing else. If there is a requirement, then HR needs to change the minimum requirements on their hiring page.