How come the Acat US Nationals is open to any International Acat competitor?
Is this common practice? How about other classes?
First off, it's not the "Acat US Nationals" it's the North Americans (international by definition).
As others have mentioned, it is common pratice to award the "Champion" status to a resident of the specific area covered by the title of the event (North America, in this case).
However, this is normally specifically addressed in the NOR. In the case of this event, the NOR is silent on the subject of eligibility for the NA Champion trophy. Hopefully, that won't cause litigation.
Mike