They're not actually the most antagonistic. It's just that they're the ones that make the most sense to see using those channels within the show. Russia and China are unlikely to be interacting with Japan intimately like that because of real life politics. The US could though considering the good relationship the US and Japan have. It doesn't make sense for Russia or China to try and blackmail the Japanese government to stop defending the inn if they don't have the channels to do so. I guess you could write Russia or China doing that but it'd be stranger to do that, I think. Besides, it's backroom politics and not public politics at play which is certainly possible unless one believes the US is a bunch of good guys that wouldn't do some backroom stuff to get things done, lol. Also, the US agents are the only ones that the Japanese interact with which makes the most sense since the governments are generally on friendly terms.
The author also wrote a bunch of reasons into the show to avoid using the US as a major player within the series. Early on, he wrote that Japan proclaimed the land beyond the gate as a part of their own. Any conflict in that case would be internal to Japan itself and not subject to the US-Japan Defense agreement, I believe. That prevented the US military from jumping in to do stuff at the beginning. It wasn't the JSDF handling the stuff in the recent episode but the other secret agencies doing it. The JSDF only escorted the girls, heh.
I feel like folks that watch the series think the really is boasting how great Japan is and is pushing an agenda. When you look at the context of what's going on, especially with the anime, I don't really think it's all that pronounced. The biggest example would be the agents that were dealing with the regiments from the 3 nations in this onsen part. The manga is a little more questionable but that probably has to do with how much more graphic it is than anything. I haven't read the novels but I do know that the print versions were rewritten from the web novel iterations by the author.
Not that it matters to much, the run from Ep 11 to probably near the end of the full run will almost entirely be focused on the other world so a lot less will be happening with Japan.