1) How were they unreasonable? Your arguement is... the terms were unreasonable... maybe? Also if the contract terms are unreasonable... there is this thing you can do called... not signing it in the first place.
If the terms were unreasonable it doesn't matter from a moral standpoint if you violate them. Morality and legality are two separate things.
2) Then they shouldn't sign contracts to do so? If you don't like your contract your only option is to sit home. Without pay until your contract runs out. If you don't like it... don't sign a contract like that! That was their options... they chose instead to violate the terms of their contracts. If you did such a thing in a sports league you would be HEAVILY fined and be forced to sit out the rest of your contract. Once again, they WILLINGLY signed away this.
So they violated the terms of their contract. Who cares? Is not obeying Bobby Kotick's every whim really such a terrible thing?
Not obeying your evil corporate overlords doesn't make you immoral.
3) Your second part exlains the first. They played activision, stalling the bonuses to make them look cheap. Now few people believe those were really coming... (even though in this very article it's brought up) anddd everyone left.
Everyone left because they trust West and Zampella and not Activision. You might want to consider why that might be.