Card issuers, Amex in this case, are a for-profit business. If issuer is losing money on you or your credit risk profile shows that it will likely do so, it fires you as a customer. That is what likely happened here.

1. Disputes. Whether by you or an AU does not matter. Those cost Amex a small fortune and require manual processing. Disputes ought to be rare in normal usage.
2. Insufficient funds - That is a huge red flag. People who write checks when they lack the funds to do so are committing a fraud and if they don't pay now, raise the chance of it happening again.

Above and beyond all of that is your entire risk profile, which seems odd and very high.

Forget Amex and move on. Next time, use AU's for the purpose which they are intended and closely monitor their activities.