I can understand why credit card debt incurred or accrued while married becomes joint. What I cannot understand is why a divorce decree does not end that obligation. In fact, it seems like your ex should be the one worried, as you theoretically might use HIS card legally (as an approved signee on HIS acount!).
My state is a community property state. Thus, when people divorce, all assets and liabilities are treated as such. which is why my ex will get a piece of my retirement when I do retire. Although I earned it, it is treated as community property during the period we were married.
Once the issue of assets/liabilities is settled, the divorce legally filed and adjudicated, that ends any further financial responsibility that is not part of the decree.
Thus, neither I nor my ex can make the other responsible for anything. We no longer have a legal relationship. No different than you and I. If you gave me signing privileges on YOUR credit card and I used it (legally) it would be YOU and not me who is liable for payment.
It would be different if it is a "joint" account. According to you it is not. I cannot imagine a credit card company contacting me regarding someone else's account.
I am licensed to give NO legal or fiduciary advice. I am simply applying a layperson's understanding of law. If I am wrong I'd be pleased to learn.
warrens