You're not missing anything. I've seen occasional races that take Visa/MC but not AX. It is up to the race director what forms of payment they wish to accept, and that gets translated into what Active.com will allow for that race. There is generally a few percent surcharge for the privilege of using a credit card, which is usually absorbed by the merchant (in this case the race director). Since the surcharge is often higher for American Express, some race directors will not accept it.

There are two common alternatives to using a credit card: Most races have an option to pay by check if you send a paper application in the snail mail. Often you can also register either on race morning or at the pre-race expo (if there is one) and pay cash.