Because if he was buzzed in or got past any metal detectors or whatever, then the school's new security measures were just proved to be major failures. Just like a gun grab would do nothing to stop sickos from doing this again. There are many unanswered questions and far too many things that aren't adding up.
Well then anyone living up in Newtown or surrounding area can put this rumor to rest by going to the school and taking pictures of the entrance for us all.