I guess several people discover 4ch after 4chan, because of the name, or they discover BBS and want to see what other ones there is. That's my case. 4ch is the bigger discussion (not images) one I found, and moves a little (unlike some image boards that are never posted on). So I came back.
The thing is there's a tendancy to post stupidities just because that's like this on 4chan. As soon as you think of something, anything, you post it before the thread disappears or someone else posts it ("in b4"). When people get here and see some idiotic posts, even a few, they'll think you can do it here too.

As for the japanese side, it's true that 4ch is very much oriented towards this, and seriously. See the "japanese culture and tradition" thread in AA. That's why I'm coming less, because I'm not really into this and don't know much.