“Left alone” is an intentional act.

Not necessarily. "The man's death left his wife alone." There's no intention in that, nor even a hint of negligence.

It seems an absolutely utterly normal phrase to describe what happened to the infant in the wake of her mother's death.

They can't say much specific about her death or circumstances because her identity and circumstances of death were not released. They can't claim "orphaned" because they don't know enough about the family. All they know in the reporting is that circumstances left an infant alone with her dead mother's body. The only thing they have is the bodycam footage of an infant and terse description of the scene. No idea if her death was particularly related to maternity. They could have tossed the word 'tragic' in there but it seems self evident.