As a full fledged member of Greater East Asia Co-Prosperity Sphere, did they have a choice?
It's hard to say how much latitude they had. I do know they had deep resentments against both the British and French, pressing them from both sides. I'd imagine that having IJA troops on their soil would affect their decisionin' too ... but there was a lot of resentment against Westerners in SEA at that time, and later, as we Americans learned too.
The Thais didn't do much actual fighting, which might represent their outlook better than any other factoid.