That isn't quite true. The issue in the other thread was the the OP wished to use a US-issued credit card with a non-US address. While certainly that should be allowed, it is not at all the same as "locking out everyone but NA residents." There doesn't seem to be any prohibition against a non-US traveler using a non-US credit card to book a ticket, and certainly the vast majority of non-US travelers would be using credit cards issued in their home country. Anyone who's ridden Amtrak long-distance trains will encounter all kinds of international clientele. Clearly Amtrak is not preventing non-North American residents from buying tickets.

To bring it back on-topic, regarding AGR enrollment for non-NA addresses, I don't really see that this is a big deal. I can see that there would be some frequent biz travelers from Europe who travel often enough along the NEC to make joining AGR worthwhile, but the tourist traveler really gets no value by signing up for AGR for that once-in-a-lifetime cross-country Amtrak trip. They'd have to come back to the U-S and ride Amtrak within 3 years to avoid losing their points, anyhow.