With all due sincerity, why do you even want to make use of 4Chan in the first place? What do they have you can't get elsewhere? I mean, do they have some wicked huge collection of anime pictures you want access to? Can't you get those anywhere else?
I believe /H/ and /P/ may very well stand for Hentai and Porn, respectively. I'm no authority on the subject, and I could be wrong, but those are (in my opinion) the most likely definitions that would occur to them. This
is the Internet, after all.
As for trying to reason out AS... forget about them. Please, just let it go. What motivates their decisions is not our concern. They did not care about Big O; they never did. Nor do they care about what else they show. I can't even explain it.
"Its a pornography store board, I was buying pornography"
Make yourself clear, man.