The person was not exposing sites that nobody previously knew about -- the sites were already compromised, there is nothing to compromise again except maybe having more than one attacker in your compromised account. The damage is already done, though.
These are likely web applications that were not kept up to date so the responsible security disclosure already happened when it was reported for WordPress/Drupal/Joomla. It is the site owners responsibility to pay attention to those security disclosures, which they likely failed to do.
And those compromised sites, in my experience, are usually attacking and infecting other sites and servers on the Internet. That makes them a public nuisance and so public disclosure is necessary so they can be appropriately blocked/isolated.