There's definitely some confusion between people seeing some things as MC because they're expensive and other people seeing things as MC because they are regarded (rightly or wrongly) as being outside the culturally not the norm among working class people even though they're cheap. Like lentils. Obviously it's all ridiculous stereotyping, but that doesn't mean it isn't frequently borne out by what people actually do!
As for the lemons... they are mainly used for easily avoidable garnish/flavouring or when cooking from scratch. Which I guess is why some people could see them as MC.