Lore: can you cite a law that that hypothetical police officer would use in that "arrest" scenario you came up with? I'm sure there is probably a federal law that prohibits the sale of pornographic materials to minors. But one would have to define a given comic book as "porn" first. I'm not saying it's impossible, but it may depend on such factors as community attitudes. I don't know where you live (and don't want/need to know), but in my area this fairly wild (IMO) hypothetical arrest scenario is so unlikely (aka arresting a store clerk for selling some relatively mild 18+ BLU comic), it made me laugh, sorry. It's identical to arresting store clerks for selling R-rated movies to a minor. Maybe it's illegal, maybe it's not, I don't know, and as you cite no specific laws (which would say clearly whether R-rated movies can or shouldn't be sold without an ID). As R-rated movies are not considered "porn", some federal porn law we all agree should exist somewhere wouldn't apply. And even if there's an official prohibition on selling R-rated movies without ID, I'm not sure if this would automatically apply to such niche thing as 18+ rated manga. It may be one of those unclear / gray area situations. Of course, anything as explicit as Crimson Spell (which Kitty basically markets as "adult book" - just look at their logo / website) may indeed cause trouble in some relatively conservative parts of the country, as Kinziechan's story shows.
As of my personal opinion on this: yaoi / any manga rated 18+ shouldn't be sold without IDs and if this were indeed true, this would probably make the whole June's censorship policy useless, and they would stop censoring (hopefully). As unfortunately this is not going to change any time soon, my feeling is that June will continue to censor, to cater to this whole "wider distribution" (=potential purchases by minors) thing.
And there's 13+ rated DMP/June's "yaoi manga" (as stated on the cover)- it's called Gorgeous Carat Galaxy.