The problem is his data may also info about legitimate foreign spying operations and info on the people involved. While there probably is still more evidence of wrongdoing in what he has, it's also likely he has his hands on something that could very well put a good deal of people's lives in danger. That data was stolen once, right out from under the NSA's noses. If the NSA couldn't stop it from being stolen, how can a single man ensure it won't be stolen from him as well? Remember, this data is very important, and he's as vulnerable as anyone to the $5 wrench decryption attack if he has it encrypted himself.
So the USA really should try to offer him this, and also offer official protection from other nations who may also be interested in some of the things he's learned. This, of course, all hinges on how many copies of the data he has, and if he's given copies to more than he's told us.
In any case, I see this deal falling through, and him possibly being forced to hand over a copy of the data to one or more third parties that are not the US, which can only end very, very badly if not handled correctly. Also, the more people handling it, the more likely it will fall into the wrong hands...