It's not just porn sites and hentai sites which do it, though, so to be perfectly honest:
[1] the answer to your question depends on the nature of the site, and
[2] it all boils back down fundamentally to Japanese xenophobia
Japan has historically been very insular even with regards to foreign powers they admire (pre-Meiji: China, post-Meiji: Europe, USA). This is not only true of Japanese property or person, but of Japanese ideas, as well. For example, Japan (as a political entity) and the Japanese (as a conglomerate of individuals) both do the following:
(a) they promote foreign views of Japan which are romantic, heroic, or otherwise admirable (e.g. ninjas, samurai, bushido, karate)
(b) they do not even mention, let alone promote, their society's fair share of flaws or faults as compared with other countries
I mean, don't get me wrong: no nation is perfect. But whereas Americans don't really do much to dispel the foreign notion that Americans are overweight, boisterous, loud-mouthed, impulsive, and often times dim-witted, the Japanese had pretty much kept the world totally in the dark right up until the Internet boom with regards to things like Japanese sadomasochism (prominently seen depicted in a nice light on television gameshows and guest talk shows), chauvinism, judicial pecularities (e.g. Japan is the only nation which places a statute of limitations on homicide, 25 years), etc. These are things which the average Japanese citizen knows about (a) because he lives there and (b) because he sees or hears about them on TV, in the newspaper, at work or school, etc. But for your average foreigner, this sort of stuff was completely off the radar until Japanese television piracy became huge in the 1990s thru the present day. The only people who ordinarily would have known, for example, about Japan's statute of limitations on homicide would have been international law scholars. Today, the ranks have swollen tremendously to include fans of Japanese legal dramas (e.g. Hero) and video games (e.g. Gyakuten Saiban / Phoenix Wright).
In the 1980s, Americans' view of Japan was "land of the ninja, land of the samurai, land of karate, and land of the atomic bomb detonations." Today, that view has drastically evolved into one which also incorporates "land of the Pikachu, land of the big-breasted cartoon girls, land of the otaku, land of the kancho, land of the subway gropings, and land of Gackt and Hard Gay." Master Splinter ........ Hard Gay. Master Splinter .......... Hard Gay. Sure, one was a rat, but at least he was an honorable martial arts master rat. lol