well yeah it was made at the request of the RSAF, but it is also operated by the 428th FS which is part of a joint US-Singapore agreement for training RSAF pilots on the F-15SG, and previously their F-16’s
i mean they are RSAF F-15SG operated under USAF squadron, so its kind of a grey area, but even then it has way more to do with US than Japan, by the known standards of addition it should go to the US
They definitely need something more than the US, but because every Asia pacific country is better friends with the US than Japan, the US apparently deserves them more as subtrees