I'm not sure you understand what Nulled means. Nulled is used when you remove all callbacks and license requirements from a theme.
I guess you want to know how these premium files end up here. Well, most of the files are purchased and share here for free.
Honestly I'm not sure anymore about that... Lately some files from babiato seems to be from another forums, and have malware/backdoors inside.
Maybe @Tomz want think about building a team to check all the themes and files before approving them (even if it's a paid one, we will donate). @Medw1311