Take this advice freely offered for what it is worth - in my own personal experience, it is most likely to be an unsatisfying experience when you enter work in an all-media or all-subject show. You have no guidance as to what will or will not be accepted, or under which criteria. You will have a much better chance of being accepted if you carefully select shows to submit to that have known jurors and/or defined themes/selection criteria announced in advance. Otherwise, it is absolutely 100% up to the whim and sentiment of the juror.

That said, what was done to you was an injustice and insulting. I would definitely write a formal letter of complaint to the sponsoring organization to let them know that this is unacceptable. Don't put any "threats" of future action in the letter - just be as clear, calm and professional with them as you have been here. But, I would also refrain from submitting work in the future, thus denying them your financial support.