To an extent they do that. I remember reading a small descirption of the arthur then under it something about One Piece being one of the best sellers in Japan (I've also seen this in Naruto).
I find it amusing that Naruto fans will pull out the canard that it's "#1" in Japan, when it's always (a relatively distant) second to OP in sales. Maybe it's just because ninjas are seen as exotic here, and therefore "cool," whereas pirates don't fit that stereotype as well in the US. But regardless, I still find it annoying.
! …Never mind that the latest Naruto chapters have been almost all fighting and conspicuously little dialogue. Despite the visual coolness of some things (like the Kyuubi finally making its full manifestation), it doesn't surprise me that Pierrot have to come up with crap themselves to pad out chapters that amount to a couple minutes of animation, tops.
reminds self to make a "ninjas vs. pirates" icon at some point in the future