It sounds outrageous right now, but...
A first-grade teacher is with her children about six hours a day, every weekday. She is in the perfect position to see the full range of the child's behaviors and moods. In many cases, she spends more waking hours with these kids than any other adult in their life. Day in, day out, month after month. This isn't the case of some stranger seeing a disturbing drawing and the words "I want to die" and freaking out. This is someone closely involved with the child, who sees their other behaviors, who actually works on and with their behavior daily, and who is a mandatory reporter.
If a kid who is otherwise happy-go-lucky did this, the teacher may well have asked about it, heard "Oh, it's from a game" and let it go. If a kid who is seriously stressed and upset because they have a parent in a war zone does it, especially if that kid has been showing other warning behaviors, it can mean something else, entirely.
Teachers don't act on their own. The admin has to approve of what they're doing, especially with something like this. I'm absolutely certain that this child wasn't the first at his school to draw something disturbing...there was something else here that caused the teacher to go to the admin, and the admin to say yes, this is definitely something we should report. Then those that got the call found it compelling enough to come out right away, rather than set up a time later, and after talking to the kid, to actually commit him.
The mother's reaction shows, to me, that she was taking the "It's no big deal! He's 6!" attitude...which is okay in many situations, but not when a group of professionals who do nothing but deal with 6 year-olds all day say, "Hey, your son's not acting like a normal 6-year-old, and we're scared that he's suicidally depressed." The article said he'd gone to therapy "in the past." Why was he not still in therapy, if he was still having serious problems? And the mom was at the school when the psych team said that yes, this child did need to be committed. She was there because she was called in by the school. I wonder how many times she's been called in to the school before about her son's problems, and how she reacts. There may be good reason why the school said, "It's not in your hands now."
What would have happened if this kid got his mother's gun and shot himself...then it came out that he'd let his teacher know he wanted to die? What if it came out that he had showed a whole range of behaviors indicating stress and/or depression (which, considering the references to past therapy, are almost definitely in play?)
I'm sure the school would be found to blame. Why am I sure? Look at the ADULT who shot Rep. Giffords. His college booted him out and refused him re-entry until he got a psych eval...and we all heard over and over how they should have done more.