Side note on this:
In 2017 when Philippine Airlines (PR) had great J class deals from AUS to other places, via MNL, we flew with them to Guam. I had checked this regulation on both the Guam government sites and on the US ESTA site and it was clear. However, at check-in in SYD the PR agent AND THE SUPERVISOR were adamant we had to have an ESTA and wouldn’t check us in for the first segment SYD-MNL.
Fortunately this was when ESTAs could generally be applied for and received within about an hour. Cue mad scrambling for the laptop and iPad as Mrs Scarlett and I applied (and for Mstr Scarlett) for an ESTA and then wore out the F5 button with stress hoping to get the response before check in closed. Even showing the PR staff the relevant websites did nothing: they were ‘computer says no’ drones and appeared they couldn’t care less. Again fortunately, the first ESTA response came through and they proceeded to check us in about 5 minutes before closing the check in desks (they never sighted the other two ESTAs…
).
Follow up email after the fact sent to PR office in Australia, as well as their SYD address, listing why they were wrong and requesting an apology for the unnecessary stress (and token additional expense) they caused. As well as hoping they’d not inflict the same on future pax. Response or acknowledgment never received.
It is effectively governed by the US, but as an ‘unincorporated’ territory it is not the same as a US State or other federal territory.