For one thing, the warez sites are constantly being closed and/or moving. It's a cat & mouse game....The moderators would have to become warez experts and keep updating the link(s).
Secondly, by posting a clickable link to a warez site, we become a "referer". If the warez site gets in trouble, this site becomes a suspect. (If it's not a "clickable" link the chances of being implicated are reduced but still exist if someone plea bargans in return for testimony.)
The administrators and moderators have plenty of forum tools for eliminating abusive users. How they use them, is up to them and the webmaster.