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.