There are lots of possibilites in English, and even though the words may be the same, the language used in law is different than what is used in the news, or in regular conversation.
For example, if you told a court you pulled up to the stop sign, and "waited a minute" before proceeding, you have just stated, under oath that you waited 60 seconds (no more, and no less) before proceeding.
Likewise, "committed" has a specific meaning in law. Being sent to a facility for observation for 72 (or 96, etc) hours does not mean "committed" in the sense of making you a prohibited person. And although a reporter is grammatically correct saying you were "committed", that's as far as they can go. By using that particular term, they are inferring, or implying something that is just not the legal definition.
Also, I very seriously doubt that Lammers purchased actual assault rifles. Assault rifles are select fire, and as such, by Federal law are machine guns. What he purchased might be assault weapons (as defined in law, semiautomatic firearms with certain combinations of specific features), which happened to be rifles.
Since he was under 21, but over 18, was not a convicted felon, nor had any conviction for domestic violence, and had not been "committed" in the legal sense (by a judge) as far as I can tell, he was not a prohibited person under the law, and therefore, had every right to buy what he did, and broke no law doing so.
That being said, even though he broke no law getting weapons, and didn't actually use them, his plans to use them consititute threats, and might also fall under some of the anti-terrorist laws as well.
Tell your Mom or your friends you want to shoot up a movie house, and its either BS and blown off, or someone gets seriously worried. Tell the cops the same thing, and its making a threat and (if even remotely creditable) gets you arrested, and the system goes to work.
I'm glad his Mom took it seriously enough to involve the authorities, because based on what they determined (as reported), he was a creditable threat to the public.
Based on the reports, he is a very disturbed young man, and we will all benefit (him included) from his being stopped before he was able to actually carry out his twisted dreams.
All else being equal (and it almost never is) bigger bullets tend to work better.