There are two possiblities going on in this situation that leap to mind.... 1)The content on the site was exclusively featuring one fetish/topic/genre of the adult industry and that fetish/topic/genre happens to be an illegal one. 2)DirectNic is yielding to complaints from someone, and decided to clamp down on the sites (which really sucks).
Not all domain registrars respond to #2 (and there are SO many ways to read that...), but some are definitely litigation-a-phobic, and therefore shut down the site at the first sniff of "sue".
I can't speak for all the registrars that choose not to roll-over, but I know at Moniker.com we prefer to give our clients the benefit of the doubt, and TALK to our client before any action of any sort is taken. (There is ALWAYS going to be someone who objects to SOMETHING on the web.) There is an exception to the "no shut down" guidline here at Moniker, and that is regarding childpornography. This doesn't mean cosplay, or a girl with pigtails where the model is over the age of majority.
I am not sure what the situation with DirectNic.com is, but I would be willing to bet that it was a combination of content & complaint.
Cheers!!