Also, given Amazon's awful reputation for building UIs that people actually want to use, I doubt they could build something to compete with GitHub anyway.
Really, I think the benefits of amazon git hosting is that they'll be more resistant to failure than github is currently.
Also I'd imagine companies with existing IAM account management structure and large teams would find this product somewhat attractive.
I haven't actually done the math yet, but it looks like it may be more economical for large numbers of private repos. For Open Source projects, there's probably no reason to use this, but for companies and other organizations who want Git hosting for private code, it might make sense. And the IAM integration actually could be useful if you're already a heavy user of Amazon AWS services.