My son is currently asleep in the other room. The baby monitor is on, but I have no idea how much noise a suffocating baby makes. It is possible that I could come into his room tomorrow to find the same thing. So it is not the same as having left him to die of starvation or leaving him in a car in hundred-degree heat. I can imagine a foolish parent thinking that if the baby was asleep and normally slept through the night, there was no reason not to go out.
That said, leaving a baby at home alone is something that should be punished, although if they are anything like most parents, any punishment will fall far short of what has already been set upon them.