Quick answer before I go to sleep.
...the guy who collect Manga did just that: collect Manga. He didn't see anything arousing in what he bought, it was just what he collected.
Not all Manga has sexual content, indeed, most isn't:
http://en.wikipedia.org/wiki/Manga
That's it. Just a collection of comics. He probably had no idea that what he was buying could even be considered illegal.
So part of my answer is that: he didn't see it as porn! It was a comic book. One of many that he collects. Just a few of them had sexual situations, they're legal in Japan, they are just drawings, and they are one of many many different kinds of comics he collected.
Simply, he didn't see, or know or even dream of them being illegal!
More to the point of your question, why do people smoke pot? It's illegal, clearly, but quite a few people do, including the people who send the pot smokers to jail.