What? There's a whole huge controversial process around trademark disputes for TLDs, spanning all the TLDs, .XXX included. It was all Slashdot could talk about for months and months, because it favors trademark holders.

It's also possible to "tarnish" a brand. So if you register something like Disney.xxx, they might be able to sue you on those grounds. There's also increased protection for famous marks, And finally, there are some marks, like the Olympics & Red Cross that are statutory trademarks. That is, they were created by international treaty and have special rights associated with them due to the legislation that enacted the treaties.

In short, it's more complex than that. IANAL, but I sure wouldn't go tempting lawyers by registering apple.xxx or whatever.