There are 2 main issues I have with this:
1. It's not "unused domains" that get this treatment. It's any domain that uses their DNS. I had several active sites and was shocked to find that blahblahblah.mydomain.com pointed to one of their spam pages.
2. If you went to spammysubdomain.realcompany.com and found a spam page, who would you think was responsible for it, realcompany.com or realcompany.com's registrar? I'm guessing almost everyone would think that the company themselves did it. By name.com doing this for active domains, they risk damaging the domain holder's reputation, for anyone who happens to stumble on to one of the spammy subdomains.