Bridget Crawford (Pace) has a rather sophisticated answer here. Pamela Karlan (Stanford), who seems to have coined the term (though I, alas, get the blame for popularizing it), offered this explanation of its meaning awhile back:
When I started using the term "law porn" to refer to the glossy promotional materials from various law schools (and I don't know whether someone else used it first and I just picked it up or whether I was the originator), I was playing off an existing expression -- "food porn." That phrase referred to a kind of breathless, over-the-top journalism about obscure recipes, usually accompanied by arty photos of food shot with annoying lighting techniques and the like. My guess is that the word "porn" was being used there to refer to the titillating way the articles appealed to the senses. Lots of people had been using that term. I was struck by the resemblances between the law school magazines and the foodie publications. Like the food magazines, the law school magazines were characterized by arty photos that often seemed designed to make the buildings or the faculty look vaguely sexy, using come-hither photos. Like the food magazines, the law school magazines used overblown language littered with adjectives designed to convey a sort of excitement. All you need to do is to look at the cover of the current issue of NYU's magazine, with its "Dworkin on Dworkin" cover, and, at least if you're in the legal academy, you'd see what I mean by law porn.
The entire point of calling the magazines "law porn" was to make fun of them, so the fact that the term seems nonsensical to you suggests its utility. At least within the community to which I was directing my remarks -- namely, friends in my faculty lounge and colleagues at other law schools -- my experience has been that the phrase communicates exactly what I intended: people instantly recognize the phenomenon and share my reaction to it.