Genre Ghettoization

Isaac at Throwing Things asks:

[W]hat is it that consigns a book to the genre-fiction ghetto?

I bought Jonathan Strange and Darkmans out of the General Fiction/Literature section ... but I picked up The Prestige and The Baroque Cycle in Sci-Fi/Fantasy. I got World War Z in Horror, but A Brief History of the Dead in General Fiction/Literature. Yiddish Policemen's Union and Casual Rex/Anonymous Rex: GenFic/Lit; L.A. Confidential/Black Dahlia: Mystery. A Good and Happy Child: GenFic/Lit; The Terror: Horror.

I can't see anything that justifies this weird classification. ... There isn't any legitimate stylistic basis for the distinction, I think (though one could argue the point with certain books). And yet one group gets to call itself highbrow; the other is stuck with the lowbrow tag (and often, but not always, the genre packaging, with the geeky graphics and the garish colors and the embarassing title font). Who makes these decisions, and why?

I blame marketing.