So you're OK with killing baby orcs and goblins then?
Yup. A dead orc today is one that cannot harm the "good races" tomorrow.
You don't know fire snakes are evil menaces. You know they have NE in their alignment box. Are you OK with killing random people on the street (in your game) because they have NE in their alignment box? Maybe they're just greedy. What alignment was Ebenezer Scrooge?
Many games are "points of light". The civilized good races hole up in small areas (villages, towns, cities), the non-good race creatures outside of those areas are generally considered evil, outlaws (outside the law), etc. Is every creature that way? No. Not in most games. Can you run a game where evil creatures are sometimes good and sometimes evil? Sure. Have fun doing it.
But a game where evil monsters are killed on sight by good PCs is totally acceptable and the game designers are totally within their rights to support that type of game.
I am not claiming that Fire Snakes should be recognizable as evil menaces right off the bat, but if I make a History check and the DM tells me that Fire Snakes are evil creatures that grow up to be evil Salamanders, my good PC is going to go over there and kill them without a single moral compunction.
Same with Orcs.
There is nothing in the current MM entry for fire snakes, or salamanders, that says that they are a threat to humans in any fashion. There is no mention of banditry, man-eating, world conquering, or anything. In most cases, the only reason you run into them is because you're in their territory on the plane of Fire.
If this is the case, then why is this an important issue? Most players will never encounter them.
You are dismissing the argument because you've already labeled them "monsters" - meaning "legitimate targets" - and don't care to consider it further.
You are supporting the argument because you've already labeled them "non-monsters" - meaning "non-legitimate targets".
They are non-monsters in your game. Not necessarily everyone else's game.