For one, because it's obviously better to have the exact voice rather than an imitation. For two, because it avoids exactly this kind of sticky situation: even if you believe you have a legal and moral right to do something, it's always nice to have the question completely air-tight.
Also, remember that OpenAI isn't a single person, but a large organization with different people and viewpoints. Not unlikely somebody in the organization foresaw the potential for this kind of reaction, knew they couldn't convince the right people to pull the voice completely, but thought they could maybe convince people to reach out again.
I mean, suppose they had a Darth Vader-y voice for some reason and had approached James Earl Jones to do it, and Jones had said no. Is it really so terrible to have someone else try to imitate the Darth Vader character? Jones, along with Lucas, made Darth Vader a cultural phenomenon; I don't think Jones should have the right to dictate forevermore all references to that phenomenon. The same goes for Scarlett Johansson and the author / director of 'Her'.