thir, Paypal doesn't want to force their morals down our throats. At least that's what I believe.
Why they feel the need to do that I don't know. The only reason I can think of is that the political and social climate in one of their key markets leads them to believe that it's possible their business might get harmed or even shut down because of their enabling smut to be purchased.
As to which key market that might be: Your guess is probably as good as mine.