The "Sympathy in Fiction" Edict.
Or its full name: "Sympathy for a belief system in fiction that parallels immoral behavior in real life does not in fact equal sympathy for said immoral behavior in real life." Or the acronym: "S.B.S.F.P.I.B.R.L.D.N.I.F.E.S.S.I.B.R.L.", or just "The Slytherin Apologists' Edict", from the group this behavior is most highly expressed in. After people who claim they agree with Mein Kampf in it's theory, but would never follow someone like Hitler because he had a stupid looking mustache on his face anyway.

Also see: ataneill93, several times.