I don't know what it takes to get a psychologist to report somebody, but I do believe there is a certain disconnect between the mental health industry and the NICS instant background check system. It's possible that many therapists are afraid to report somebody for fear of losing their job or a client, but on the other hand, some people who are legitimately nuts may know how to hide/mask/camouflage their intentions. All you really have to do is not commit a felony (or misdemeanor domestic assault, if I remember correctly) and things will generally go smooth if you want to go the "buy new" route. As long as you don't commit crimes and keep your mouth shut about your plans, seriously, no one's going to be the wiser until it's too late. Of course, this would not have prevented him from buying used from a private individual, or even stealing from someone he knew. And good point bringing up knives as well, this is the part the media is practically ignoring.
Mr. Coffee Tosser certainly had far worse problems than simple autism, and people did pick up hints that he was off, so I'm chalking this one up to a lack of communication between the therapist(s) and the proper authorities. Besides, even if a motion was made to commit him involuntarily, it is very difficult to do so due to patients' rights, and the burden of proof is rather heavy on that. He needs to be proven a danger to himself and others beyond a certain level of doubt, and even then, if he completes treatment or doesn't show any signs of being insane or whatever when being kept in the mental ward, he can get papers to prove he's clean and then go on anyway.