"They could have stated up front..." - IIRC their ToS stated up front that you can't accept that money at all (i.e., no pre-sales) - but they didn't catch you in time, so they are holding the funds. It's essentially as an escrow, but better than that - escrow would release funds only AFTER shipment, but they are presumably offering to release part of it before shipment according to the requested business plans. And they have no obligation to catch 100% of violating merchants immediately, it's your responsibility to not request payments breaking the ToS.
"They could have demanded the backers to waive their refund rights." : No, they can't demand that - the refund rights are mandated by law and nothing can waive that, consumers have them no matter what any merchant (like you) or intermediary (like paypal) might want or put in agreements; some chargeback rights apply even if they sign in blood "this is an nonrefundable donation".
"They could have demanded the backers to waive their refund rights." : No, they can't demand that - the refund rights are mandated by law and nothing can waive that, consumers have them no matter what any merchant (like you) or intermediary (like paypal) might want or put in agreements; some chargeback rights apply even if they sign in blood "this is an nonrefundable donation".