LH ETIX is not limited to Amadeus users. It can alos be ticketed using Sabre and Galileo. It is supposed to be relatively complicated with Galileo. It is the standard procedure for Amadeus, you have to explicitely override it if you want to issue a paper ticket. I suppose Amex Corporate is just to lazy to do it. Or they have special discounts and these tickets can't be ticketed via ETIX.