Cannibalism is eating members of your own species. Rumia eats humans, but she's not a human herself.
A non-human who eats humans is not cannibalistic, it's anthropophagous. Or, for simpler terms, at least use 'man-eater' or something. But not 'cannibal'.
You forgot the Danbooru rule: "Tag what you see, not what you know." Meta-knowledge, aside from artist's commentary and in-picture text, does not apply to tagging.
Anybody who stumbled upon this pic with no prior knowledge of Touhou would only see a creepy little girl eating human flesh. They'd think it was cannibalism. Thus, the tag.
You forgot the Danbooru rule: "Tag what you see, not what you know." Meta-knowledge, aside from artist's commentary and in-picture text, does not apply to tagging.
Anybody who stumbled upon this pic with no prior knowledge of Touhou would only see a creepy little girl eating human flesh. They'd think it was cannibalism. Thus, the tag.
Im sorry but i feel that is a flawed logic. by that logic anyone who see's a character like lets say Astolfo from fate as a example. because if they stumbled on a said characters pic they would just see a really cute girl with pink hair in some sort of knight outfit not a fem-boy that is a high ranking paladin and also is it really that hard for people to ask about a character there not familiar with in the comment section or look at are wiki?