This is one of my gripes about the BJCP/Craft/homebrew judging.
Diacetyl isn't evil, and should be found in some styles. I don't have any data on it, but I'd imagine that some oldschool RIS beers had diacetyl, being brewed in England after all. I know the BJCP says "no diacetyl" for RIS, but I don't know that I agree. I feel they too often lean towards describing American interpretations of styles, rather than the actual beers themselves.
Case in point: Northern English Brown Ale. "Very low to no diacetyl".
Ever had a Samuel Smith's Nut Brown Ale? This beer is often used to show folks what diacetyl tastes/feels like. There's nothing low about the diacetyl in that beer, and in fact it's an important part of the beer's flavor. Even the BJCP listed this beer as the "second best" example of the style in their old guidelines, but yet "low to no diacetyl" is appropriate for this style?
Granted, all this is just my personal opinion, but this is why I have zero interest in the BJCP or competitions.