Since the funds can only go to one party there is little opportunity for fraud. Likely you can only pay once at a turnstile (or station for that matter). Not sure how OMNY works but usually transport passes will not work unless you've checked-out before using it for another entry.
The pass is only an identifier, it contains no payment information (you could be using pre-paid, credit card, direct debit but that happens on the "server" end).
