Hm according to Wikitravel travel between mainland and Puerto Rico is considered as travelling between two mainland cities.
http://wikitravel.org/en/Puerto_Rico
Maybe it has something to do with that Guam has its own visa waiver program? Well I still found it funny that even US military personell has to through CBP even though they arrived from some remote base...
So let's say I travel to Guam on a valid US student visa from HNL they take my I94 right and I have to complete a new one for Guam and also when coming back?