The TC1-etc. rule plays a part but it's not the reason. The key here is the way the ORD-DEL rule is worded: you have to pay for Europe regardless, even if you never touch it. After you pay for Europe the TC1... rule then comes into play in that it prevents you from going back to Europe to 'use' it.

IMO AA framed this rule deliberately to discourage OWE users from taking this flight.