Joining a losing BG is mostly fine if it ends fast, you get some quick honor for nothing. The problem is only when you wait 15 minutes or more to join it - in that case you should be rewarded by getting priority in queue for your next bg.
That would be definetly better than just adding surrender option that would be abused and just cause more problems overall.
But to be honest random bgs are a joke and 90% of them are a stomp so...