That is a point but does it relate to a nation or a government? If elements in Thailand approach the allies offering to basically change the government and switch sides, which would help the allies in the war in SEA, what would the reaction be?
It pretty much applies to the institution of the government, not just the individuals who happen to be filling the roles, not just to, with a nice big dollop of racism to apply to the people as well. This is based on what the British wanted to do iOTL, when the Thais did replace the government which sided with the Japanese, and the British still wanted to treat Thailand in the same way Japan and Germany were treated, i.e. the original institutions of state dissolved and replaced with an occupation government, pieces carved off, and other regions placed under indefinite military control.
The US believed that the British were deliberately prolonging the war with Thailand in order to accomplish this, and they were probably right. They also believed that the British were doing their best to interfere in attempts to get the Thais to change sides...
Given the current military situation, the Thais changing sides doesn't actually help the allies in any significant way - the main Japanese limits on attacking the British are local logistics, so having to occupy the rest of the country doesn't really compete with that. Given Thailand's extremely poor internal transport links apart from railways laid to transport timber from the interior to the coasts, an allied Thailand doesn't even help much with liberating FIC - it's simply so much more efficient to ship men and supplies directly from Singapore compared to having porters and mules carry it overland.
British strategic interests are best served by militarily defeating Thailand, and imposing a military government on it as soon as possible, including annexing the southern provinces, to create facts on the ground well before the end of the war.