United's Contract of Carriage are incorporated into each and every fare that they sell. The terms and conditions of the TravelBank are included by reference when TravelBank credit is used to pay for a ticket. You're looking at the wrong type of law -- you need to consider contract law, not criminal law. In this case, UA agreed to provide a service -- namely, your transportation -- in exchange for certain compensation. As you did not provide valid compensation, you've incurred a debt, and United has attempted to collect.

A better example would be if you had used a stolen gift card or counterfeit currency to make a purchase. In such a circumstance, the seller has the right to be made whole by the buyer. You can't return the ticket, so UA's only alternative is to attempt to collect cash.