I would expect he is under no legal obligation. But he ought feel a professional obligation to history that others may be able to use the recordings/transcripts to draw their own conclusions. He is a biographer, after all; he never got to interview Einstein, he had to rely on the records of others that were preserved. That doesn't mean the transcripts must be released immediately, but I would expect that others will eventually have access.
How often does this actually happen? I'm not altogether that knowledgeable in this area, as I am neither a biographer nor a historian. But I don't know where the "professional obligation to history" comes from. He's making a commercial product, not doing a research paper for a public university.
And sure, it'd be nice to see his source material eventually. But I'd rather see the coherent package of his research in a narrative book form rather than his notes. I'll trust Isaacson to tell the story of Steve Jobs through his own interpretation, as that is all that a biography can be.
I'm not an expert, either, and I don't know of much precedent for interviews like this with an individual author being publicly released (say, on the web, rather than in a university archive), but private archival materials are used by historians and biographers all the time. I don't know whether Isaacson or his publisher own the rights, but I'm sure others will ask for access in the future.
I'm not sure about characterizing a biography as a "commercial product"; although although it is the source of the author's livelihood, and the publisher is a for-profit entity, it is a complex entity as a product. The author depends in this livelihood in having access to materials from others, it is not unreasonable to think he may reciprocate in some circumstances. In any case, in future years (after the initial sales peak of the bio), releasing the source documents may even cause a revival of interest that helps residual sales.
I would assume they're actually under obligation to not release them. Most people that sit down for these things would rather every word they've spoken does not see the light of day.
