I thought one of the reasons people incorporated companies in the US is that there is a working judiciary system that can ensure the upholding of contracts. Sure the big money can apply some pressure to the dispossessed but if you have a few million cash (and OpenAI surely has) you should be able to force them to uphold their contracts.
Also imagine the bad PR from Microsoft if they decide to not honour their contracts and stop OpenAI from using their computer power for something that OpenAI leadership can easily spin as retaliation.
Sure, this latest move from OpenAI board will wreck the momentum that OpenAI had and its ability to continue its partnership with MS but one of the thesis here was that that's the goal in the first place and they're legally free to pursue that goal if they believe the unfolding of events goes against the funding principles of OpenAI.
That said, they choose a risky path to begin with when they created this for-profit controlled by a non-profit model.