The biggest charges were ordered to be paid by CNEN (since preventing this kind of accident is part of their job, and they were notified of the potential danger well in advance) and IPASGO (the new owners of the site, who were also aware of the danger and didn't really do anything about it). Finally as you say, two people from IGR (the clinic that moved out) were also charged, because they removed "gates, windows, timberwork and the roof" from the facility while leaving the radioactive device for later - which might also explain why the new owners eventually got pissed and decided to lock them out.
In short the whole thing was a collective train wreck, but the most obvious failure seems to be that CNEN should have taken action immediately after being notified of the danger. Hopefully they're a bit more reactive now.
https://web.archive.org/web/20120318233148/http://www.oecd-n...
Lack of education led to the trespassing of property, zero knowledge of radiation to believing it was supernatural, the justice system preventing the cleanup, etc. Means there is no single point of guilt and its hard for people to grasp this.