I have no experience with this act, but a quick search produced many references--this page, for example:
https://www.onlinegamblingsites.com/law/uigea/ It says UIGEA does not define the term "illegal gambling." When that is the case, you are left with a general obligation to follow commercially reasonable practices. Guidance from banking regulators and trade associations would be very helpful, and you've probably looked into these resources already.