Forgive me if I can't help think of "sin" and "cos" when I think "tan"...
But it seems like they like affixing it to any female, and sometimes to some males (in jest). Heck, they even use it for cats... (I visit /27/ on 2chan regularly) cats seem to have quite a cult following, for example longcat [nobiiru-tan] has even appeared in a lot of other jokes and
neta and what not. Even in English places like 4chan. Guess you can't deny the appeal of.... a long cat!!
I would find it really hilarious if I bumped into a discussion on 2ch where they talk politics and then they go on about "Laura-tan" and "Merkel-tan" and so on
Though I have to say, Afghanis-tan really was a smart idea...