You really should get in touch with a lawyer before trying to argue about what you clearly don’t understand. Regulatory framing doesn’t have anything to do with what something is or is not classified as.
Curve is a merchant, amex blocked it’s payment scheme from being used by Curve. It’s the same as if Visa or MC did it. If Amex had blocked only cards issued by them from being accepted by Curve, like any bank can do, then they would be in the right