There are lots of other possibilities.

Here's an example: let's say you had a business Centurion, and Amex finds out that the nature of your business is a cannabis dispensary. Even if you never charge anything that the federal government would consider illegal to the card, and even though the business might be fully licensed and compliant with state law, there's still the fact that the card is being used to operate a business that the federal government considers illegal. I'm sure others can come up with other hypothetical situations that have nothing to do with credit worthiness, transactions, or payment history. The fact that the letter from Amex said "... we have cancelled the account(s) based on adherence to regulatory guidelines" makes me think the issue is more in the neighborhood of what I said above, rather than a traditional creditworthiness issue.