I wouldn't say anything. I would walk away, as Zilla proposed, but with even less explanation. If I was attached to the cause I would find another charity in the same area, and work for it.
This was handled so poorly from the get-go, and you were treated so dismissively, OP, I have serious doubts about this outfit's ability to handle money -- as well as their beneficiaries.
What about this suggests they are bad with money or their beneficiaries?
Yes, I'd be careful of making that leap. Whether it was a mistake, a miscommunication, or maybe a political thing (maybe the group's biggest donor saw rival's poster design and fell madly in love with it), that doesn't necessarily mean the organization is a mismanaged mess. Believe me, I'm involved with organizations that run like clockwork but have alienated many volunteers, and others that are seat-of-the-pants run but treat their volunteers like gold.
It was actually an organization in the latter category in which I got into a situation similar to the OP's. They were putting on an event and were looking for a clever name for it. I came up with one, and when it was passed on to Woman in Charge, she loved it. I think she even said something like, "We have a winner!" When the event was officially announced, it had a different name.
I was hurt, but I figured one of two things had happened: She'd forgotten my idea, or she'd thought it was a joke (it was kind of out there). I may yet walk away from that organization, but for reasons totally unrelated to that incident.