The problem is that Amex does do partial disputes for US card holders. It is an express choice on the webform.
But, in countries where that is not an option, it all becomes confusing.
For the benefit of others, while US law only requires that card issuers provide chargebacks for 60 days, Amex and many other major card issuers will process a chargeback for up to a year. It is generally a poor idea to touch the payment terms until after the ticket has been completed. Nothing good comes of it and this may just be one of those examples.