I got this reply from Lego customer service today and thought I would share it. Note that they tell me later in the email that I might be able to buy it from a third party seller through Amazon. Interesting.... Confusing policy, Lego! It sounds like Lego wants third party sellers to fill in their gaps.
Thanks for getting in touch with us.
We appreciate you taking the time to share with us your disappointment with our LEGO® Star Wars™ planet sets. We know that these were very popular, and do apologize that the next series will not be sold in North America. Ben, where did you see information regarding the next series? If your region is selected to the United States while browsing LEGO.com, it should only show you items that are available for your country. If you do a standard internet search for an item, it will direct you to a LEGO page that has the product available, which could mean showing you other countries.
We work very hard to make sure we are always launching new things all around the world, and sometimes some products do not make it to each country that we support. We can assure you that there are products that can only be found within the United States. Unfortunately, this means that you will not be able to purchase these items through us directly and have them shipped to the United States. You may be able to purchase the next series from a third party website, such as Amazon.com. We are more than happy to get your comments passed along regarding these sets to our team in charge of this area. We always want to make sure we are providing the best service to our LEGO fans. To see our new Star Wars releases, please see the link below: