You misunderstand; they're free to add any event they wish, including the backwards triple jump for crosseyed midgets, but the operative word there is FREE, which is what the world's top milers won't run for.
And if Oslo (no longer one of the obscenely big-money meets on the circuit) can't afford to add another high-priced field they won't.
Becuase of the attention the U.S. media (partic. ABC-TV forall those years) has given it, the Dream Mile enjoys far greater status here than it does at Bislett.
But fear not, it's still going to be one of the great meet-going experiences of your life.