I know many people holding Centurion cards from back in the day who barely use them, I have never heard of a Centurion being pulled for low usage and $500k is not exactly peanuts anyway.
Would you be able to share any more details from the letter and your usage of the card? Biz or personal? Do you have any other Amex products?
I have had a hit and miss relationship with Amex over the years but never quite this bad. Has to be something more going on?