I have the feeling, because then the earnings could be detected by tax offices. I will not judge anyone, but avoiding taxes is hardly in the responsibility of payment processors or camsites.
You know, each country is different. In some countries camsites/adultsites/porn/etc are in the "grey area" (not explicitly legal or illegal) or even prohibited. So, banks might flag and report it(for this kind of wire transfer) on so many levels, so the taxes will be the last thing you care about.