The intention is for "No Characters" to match the effect in Yuletide of selecting zero characters in your request. That is, it's a way of signalling that you don't mind receiving zero characters, not that you *require* zero characters.

If a recipient did mean to say that they wanted something that was character-free, they would need to either select only the "Meta" category of fanwork to receive, and/or put "characters appearing" in their DNWs.