fraud (frôd)
n.
1. A deception practiced in order to induce another to give up possession of property or surrender a right.
2. A piece of trickery; a trick.

I think that is clear, and the T&C were clear too (even "hidden"). If they consider you make fraud, they can cancel it . Law do not protect ticketing in this situation as the website do not show any fare error, only when you applied a voucher on payment.