Any power not at war is neutral, not friendly.
It doesnt say that anywhere in the rules or the errata.
Nor does it say this anywhere.
US and UK/ANZAC are referred to only as friendly as they inevitably WILL be the “Allies”, while Germany/Japan are friendly powers as they inevitably will be the “Axis”. But if a power isn’t at war, they are not yet aligned, and are as such “neutral”
Here are some things that it does say.
The powers that begin the game neutral, the United States, the United Kingdom and ANZAC, are initially not considered to be fully part of the Allies. While Japan is considered to be on the opposite side from these neutral powers, they are not considered to be enemies. While they remain neutral, these powers have some special conditions and restrictions on what they can and cannot do.
It says the US, UK, and ANZAC are neutral, and not FULLY part of the allies. Then it says that Japan is considered on the opposite side of these power inferring that the US, UK, and ANZAC are somewhat aligned even if they arent fully part of the allies. It doesnt say all of these powers (US, UK, ANZAC, and Japan) are all neutral and on opposite sides even though they arent enemies.
Powers that are not at war may not move units into a friendly power’s territories or onto its ships, nor may friendly powers move units into its territories or onto its ships.
If there is no such thing as a friendly power while a country is still neutral then there is no reason to specify friendly power in the above statement. Simply saying Neutral powers cant move units into other territories or ships and vice versa. Instead it says friendly powers indicating that such a thing does exist even to a neutral power.
Now, you might be right and the US cant use the naval base (I am not sure which is why I asked the question); however, nothing in the rules or errata says that it cant and statements similar to the ones I posted above suggest that they can. If somebody sees something I don’t then please let me know. Right now everything in the rules seems to suggest that the US and UK/ANZAC are friendly and thus can enjoy the benefits of naval bases.