I can kinda see the reason why DMM can offer Paypal and R18 not, because DMM also has all-ages content that you can buy with those points. Another site that has this loophole where you can use Paypal for points and use those for adult content is DLSite, because they have lots of all-ages content as well, whereas R18 is nothing but porn, whether you're buying with those points or not, but still...
So I think the solution here is for R18 to buy some cheap japanese drama series or something that nobody gives a shit about and just offer that on the site. Then you can always say "Oh, those points are intended for the regular TV Shows!".