If fires didn't trigger hostitilies, you could use molotovs or exothermic aura to assassinate pretty much anyone without any consequence.
I agree with that - if the AI got caught up in your "effect radius".
But if the fire is already laying there on the floor, and an otherwise unaffected AI chooses of their own volition to step in (or through)it; then it seems like it should be "on them" for voluntarily doing so; and thus not start hostilities.
After all, it could be around a corner where they didn't see you throw and didn't know there wasn't already a fire there.
Might just be a quirk that we have to accept - but to me it logically makes sense that way.