Hi Nicolas,
I did some further testing and here is what happens:
First of all I need to say that I need the t&c to be part of registration process rather than checkout.
Right now the t&c check box appers twice under the rego form and on the checkout page. When a user registers the t&c is not validated. But it is validated as part of checkout process, here I mean if I try to proceed to payment step without selecting the t&c box, I get the expected error message.
I hope this helps to understand what's going on.
Thank you.