Don't torture yourself about the word "international". HNL-GUM is domestic and GUM-NRT is international. But, UA is free to define these terms for its own services as it chooses. All kinds of carriers do that with services within North America, e.g., US-Mexico and the like.
The rationale for why UA does not do freebies on HNL-GUM is simply supply & demand. Same thing with as to why UA serves 2 Y meals on EWR-LHR, but none on EWR-LAX/SFO and so on.
Despite OP's two experiences, the route is one where there are many businesses which will pay for premium cabin Mainland US - TKY, but with a cap. Because Mainland-HNL-GUM-NRT is cheaper than the various non-stops (and service standard lower too) those with caps will pay for premium using the slow boat only. No reason to compete against yourself by giving away what you can sell.