Yeah, no one can force you to enter or even compete. I think, if it were me, and I was trying to exhibit but the director/con staff insisted otherwise or didn't want to make allowances for exhibition (all competing or nothing) then I would politely refuse and walk away. I don't need to be on stage, particularly with something I've already won, so if I'm floating around some tiny convention that barely has a structured contest, I would probably give it a miss no matter what. I don't consider it arrogance or elitism, it's simply a case of facts: if your skill level is light years beyond anything else anyone at the con is wearing, after honest assessment, it's only polite to step back and let some young newbs have their night in the spotlight.
Nothing says you can't go for it anyway, though. It's sort of a catch-22, isn't it? Sometimes entering at these small cons can help raise the bar so that in subsequent years, people feel like they want a challenge and will bring their A game. But I would agree that if you happen to be wearing a costume that has won major awards before and either weren't planning to enter or weren't planning anything beyond an exhibition just to amuse the audience, it's more polite to refuse offers to compete than to say 'well the director said it was cool so take it up with them.'
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com