The difference is the issuer. With an AMEX card from AMEX, (like a amex prg or a BCP or something) you're just getting a card that is both issued by amex, and part of the amex network. If you get a USAA amex, you're getting a card that is on the amex network, and then issued by USAA. It may have a different reward structure (I don't think it earns MR points and stuff like that), but (AFAIK) the benefits of the card should be the same. Like 0 liability protection, roadside assistance and stuff like that.
Hope this helps
AMEX is a network. There is also an AMEX bank. Most AMEX network cards are issued by AMEX bank, but this one is issued by USAA. Both the network and the issuing bank (along with the merchants acquiring bank) get part of the fees charged to the merchant when you swipe the card, with most of it going to the issuing bank. It's the issuing bank that is on the hook should you not uphold your end of the bargain and pay them for a transaction.
Cards in the other networks (visa/mastercard) are issued by many different banks. Discover network cards are issued mostly by Discover bank but some (like the Walmart card) are issued by other banks like GE.
Backdating does not apply to network cards (USAA,BofA,Macy's).