Players, on the other hand, are mercenaries. Willing to deliver books, torture natives and dig through poop for a few gold coins. They are also so frighteningly powerful anyone with a brain (aka quest givers) would turn a blind eye to race to keep the other side from getting their services.
If the Horde is having a civil war, the Alliance can just hang back and eat popcorn while they plan to attack the winner. Meanwhile, Bob the Human can sell his sword to Vol'jin's rebels because he is a mercenary and gold is gold, and Vol'jin doesn't want him working for Garrosh. No need to stupid diplomatic missions and forced inclusion, let things play out naturally.