I would not read some evil motive into Marriott's failure to respond to Amex. The fact is that most merchant vendors don't bother and the chargeback becomes final. Those who care about it simply show the item as a debt and if you happen back may demand it (Amazon & Fedex are good examples of places which largely shut one out if there is a chargeback filed and sustained).

Sorry about the spam filter, but it is important to look for confirmation emails and when they don't arrive, to track them down. In this case, you don't know whether the confirmation really was at the higher rate.