If I wanted to hand user data over to a 3rd party that tapped the entire backbone, I'd make that user data unencryptable. Why would I want to send Gbps of traffic to that third party? Then everyone would know. If they can just analyze the recorded traffic, none has to know.

Intelligence agencies want as many possible vectors for attack as possible. Especially unknown ones that you are not prepared for them to exploit. Everyone is assuming they wouldn't bother with a client backdoor... That right there is enough reason for them to get a client backdoor!