You may be right on the payment methods as a reason for there being less of it. I think I need to clarify the filter point. It's the same filter, but I think the filter is easier to bypass if the spam is in English. It's the same things being looked for, but the keyword search is weaker when the person uses the mish mash of special characters to screw with it. I have no doubt that if they thought it would bring them business, they'd find a way to screw with the JP text search too, but if the market is smaller and people come to you....why bother?
There is the perception among the EU and NA players, and I don't think it's wrong, that SE gives a lot more love to the servers in it's JP data centers. It's no different than Blizzard giving more love to it's US servers over its EU and Brazil data centers. It's where the devs play. It's their patch, so they weed it better.