Heiko Hartwig I can see how the system can be abused. And I can totally support them in that they try to defend their product. And no Credit cards can not be used.
Apart from discrimination, they have a policy that your first pair of boxers is free. With a maximum of 1 per person. If people play the system by making new bank accounts and sending in fake names then that is something where OTA needs to look at, not the payment methode.
for comparison, services like Netflix have the same issue. Their first month or so is free, and people with multiple credit cards can simply make new accounts. Companies like netflix dont start blocking banks, they start actively checking new customers if they are really new. Simply onle one trial per address for example.