The "backdoor" is that the receiver has no privacy or has to trust the sender. But thats by public design .. its a public announced feature of the software. Thats not a secret backdoor .. a public fro...
Its different from Bitcoin, sure. But in a normal western world where every commercial merchant needs to disclose received payment thru detailed booking to the government anyways its a common model they replicate - tranparency on the merchant side (receiver) and perfect privacy the customer side (sender) is a valid compromise for an commercial payment system. As long as Bitcoin for private to private payments is not illegal on the side .. I have no problem with that.