As an agnostic with no "horse in this race", I have to say that I have no clue why Spong and people of similar beliefs can't just leave their Church and found their own thing, when it's pretty clear they don't believe in even its most basic tenets.
You are making the same argument as the "'murica ain't a socialist country, and if you dun' like it, you can giiit out!" people.
For a second I was surprised that you'd come to Spong's defense, but then I realized that you probably agree with the "'murica ain't a socialist country, and if you dun' like it, you can giiit out!" people.
Anyway, States are universalistic political communities whose responsibility is to serve the interests and abide by the will of all their members. While I'm certainly in favor of Churches being responsive to their membership, as private organizations, Churches have the right to set their rules and principles irrespective of individual preferences (within the bounds of the law), and it makes sense for someone so radically dissatisfied with the basic principles of their Church to go their separate way.
(Again, I'm not saying that every disagreement should end up in a departure, far from it - most of my family are Catholics despite disagreeing with basically all of the Church's politics and rarely even attending Mass, and I have nothing against that at all. I was only talking about militant revisionists like who are demanding their own Churches to abandon everything they stand for.)