How is it a gray area? It quite literally say you are not to gain monetary value from not using it yourself.
How solid is the legal basis for putting "not for resale" on things like this, and how enforceable is it? What's to stop printing "not for resale" on physical copies of games? Is it one of those things that if challenged would not really hold up, but it would just not be realistic to do so? It's interesting to me how upset people get about attempts to shut down/subvert physical re-sale of games, but many seem totally ok with not completely realistic restrictions on a secondary market for digital copies of games. This is more of a general question, and not directly to your post.
Also some people seem to calling the codes given to them by Amazon stolen, but I don't see that as technically accurate seeing how Amazon actually provided it to people before their order was processed. Obviously, the customer intent was not above board, but it was Amazon's goof to provide them before an actual purchase was confirmed and shipped. The codes themselves were not fraudulent, meaning they weren't fake/counterfeit, just that Amazon made a mistake by giving them out.