Originally Posted by Henwurst

Yup, Guam is a US territory just like Puerto Rico.

It can indeed get confusing sometimes, but it is fairly common that different parts of a country have different rules, often to facilitate trade or because the local environment/agriculture is extra sensitive. Hawaii has separate customs from mainland US and Hong Kong has its own immigration, customs and currency even though it is part of China. In the case of Guam and Saipan they simply have less restrictions on citizens of nearby countries than mainland US, which I think makes a lot of sense.

Chinese and Russian citizens have very few restrictions for visiting Saipan which has helped boosting tourism.