So how would you avoid commanders from abusing this ability? Lets say that an alliance completes the event with everyone playing evenly and keeps on going far enough that they only need points from 19 players. At this point the commander is in a position to threaten to take away the prizes any member has helped earn. Or alternatively they could kick a random player to let a friend join in their place to take the prizes after completing a side node.
That solution seems worse than the problem it claims to solve. Again: you've still got the option to not play with the person in future events by kicking them from your alliance. Or if you want to vet new members, you can set the alliance to private.