They knew he was going to leave. It's likely a combination of the following:

1. They couldn't fire him as an employee (or felt it was an overreach of their mandate)

2. They wanted to signal a clear distinction that they lost faith in him as Chairman, while not losing faith in his work as an employee.

3. They felt like it would play better with the company if his ultimate departure was his decision rather than theirs.

They don’t have to pay him an exit fee.

I doubt they care about this. This move already signals they're not optimizing for financial outcomes, and the independent board members (3/4 involved in this decision) have no equity in OpenAI.