Do you click OK to a warning banner when you sign-on to the network?
Although porn is blocked at most places there are still some places you can access e.g. FF or emailing. And that is enough to fire someone if the company policy says that. Also some users or carry porn on CDs to work. Another no no. These web sites blocks are in place not just to have employees concentrate on work but also protect employers from lawsuits, HR issues e.g. sexual harrassment if a another co-worker walks by and see something he/she does not like seeing at work. Trust me these things have happened.