You say players aren't allowed to leave? No one can prevent someone from quitting the application or shutting down their computer. But I'm guessing that the service won't let you join a different game until the current one finishes...
Is there a way for a team to resign? Surely if a team unanimously thinks they've lost, they can surrender immediately...?
On the other hand, my worst experience with Dota was that a lot of players were leavers, so any sign of losing and someone left the game. And after that it's really game over. No fun to lose the game this way, even no fun to win the game this way. But with HOTS that's completely different. Almost everyone tries to stay until the defeat and it's not that rare to see a comeback and victory.
So I'm supporting that notion of not allowing anyone just to leave. It's not a chess and there are other players who might want to fight until the end. On the other hand, quick anonymous poll, initiated by one player, if anyone wants to stay, and defeat if everyone wants to resign wouldn't hurt anyone, I guess.
Your team can resign, but it requires at least a 4/1 vote. You typically end up with that one try-hard player who refuses to end with dignity and get 3/2 non-resigns, forcing everyone to play out a lost game.
On top of that if you afk or troll yourself there is a reporting feature at the end of the game where you can flag players, which can also result in account suspensions and bans if they occur too often.
It really is a terrible system all the way through.
* A simple majority vote for designating a player 'too weak for this level of play'.
* A strong majority vote (>75%; in a team of 5 this would be 4+) for designating a player 'a troll'.
* Either of the above ejecting the player from the team at that instant.
* Allowing all other votes to proceed as normal (N-1 required to resign a match).
Now add in its RTS roots - an overhead God's eye view lets you watch your teammates. RPG elements means a bad player isn't merely useless, but he's actually feeding power to your opponents.
So a long game, with a prolonged endgame that often results in 10+ minutes of moribund gameplay after the match is prettymuch already decided, without even the dignity of a quick execution like an rts generally provides. No leaving allowed, either. And you get to watch your teammate (you only have 4) who brought this fate on your head, and you're stuck together for the whole round.
They couldn't have designed a game that makes you hate your weakest teammate more if they tried.