I feel the same way you do, I do think the only reason no punishment is fear of repercussion of losing MORE subs which means less money for Bioware.
That is the bottom line, but it still by no reason makes a good enough excuse not to implement it.
I'd rather let people quit than have them AFK on the defender node. If anything, I'd only allow 1-2 backfills and if more than that many people drop then it should stop adding people in and let the match die. There's about 20 things that need to be done to prevent people from leaving but stretching on a lop sided match to the end is just painful for everyone else who hasn't quit and is really a lose-lose situation for everyone on that team (and an easy guaranteed medal farm for the enemy).