An American Express spokesperson contacted TechCrunch to say Google had never "gotten our approval for the additional use."
Now, that doesn’t mean you actually can’t add your AmEx card on Google Wallet. You can, but it’s just not officially confirmed that the whole thing would work after a while. Google has responded saying that “active discussions” are taking place.
Folks at Google also said they "look forward to working together as partners as the world embraces digital payments."