The thing is that if US asks Japan to directly fight in an attack on China, this is tantamount to asking all of Japan to give up everything they own and die. It's a very high ask.It's not about having the power to eject the US. It's about whether or not it sets off alarm bells in DC. If some small unimportant country in Africa wants to end military cooperation with the US, the US will follow the law because it doesn't matter. If a country like Japan, Korea, or Saudi Arabia signals that it wishes to leave the US orbit and enter China's orbit, that will set off huge alams bells in DC. The terms, "concerned about the democratic situation" will start to pop up in US media regarding that country and the US will resort to sanctions, assassinations, color revolutions, election-rigging, etc... to stop it from happening.
To the US government, when the cards are stacked and the stakes are high, a "sovereign government" is one that sides with America and a "tryanny that no longer represents the will of the people" is one that opts to leave America's orbit (particularly towards China's). America has absolutely no respect at all for Japan or Korea; they might be powerful countries compared to the global average but they're small potatoes compared to the US. They're either going to stay America's stooges or they become America's enemies and they can't afford to become America's enemies.
Mohamed Bin Salman is sick of America's shit; but he's still not going to join BRICS and he's still going to play along with the Petro dollar. He can sign more deals with China because that's salami slicing that the US can tolerate but if he plays power moves to China's tune like ending the Petro dollar and joining BRICs, the US will make him one offer behind closed doors that he can't refuse and that's to know his place or end up like Saddam Hussein.
When everything is fine, it's easy to be anti China and pro US when it doesn't have any price. It's like how there are billions of Christians, but how many actually lay down their possessions to follow Jesus? Actually scratch that, how many even pay their tenth?
There is no such thing as a real all weather ally, it's all about costs and benefits. What do you think Belarus would have done in the Ukraine war if Ukraine was 3 times larger than Russia, had air superiority, ability to hunt down the whole Belarusian air defense network within a month, can put thousands of ch5 style drones above Belarusian cities and have tens of thousands of rockets pointed at Minsk? Think they would still lend out bases to the Russians?
I don't fully know if Korea and Japan can afford being sanctioned and assassinated by US, but I know for sure that they cannot survive a war of aggression on China.
Don't forget that as of today, Japan and Korea are not China's enemies, if they were, they would be dealing with much larger problems.