But by its inclusion (as a juvenile salamander), it brings in the Unfortunate Implication. It reinforces the unpleasant stereotype of D&D as a game where everything is a foe to be slain, and the players as relentless "murderhoboes".
Does it?
As you yourself pointed out, nobody would have cared if they would have left Firesnakes out of the MM.
I opine that nobody outside of our gaming community would have had this stereotype reinforced because they too are nearly all unaware of the existence of Firesnakes in 5E.
In other words, if people such as yourself had not brought up the issue, even people inside of our gaming community would not have been aware of it, let alone people outside our community who might stereotype us.
Now if there were hundreds of such creatures in the MM, then you might have a point. People both in and out of the community might eventually notice.
But for a small handful of monsters? Is that really worth anyone getting their panties into a bind over, and further, isn't getting ones panties in a bind over that exacerbating the very stereotype that you are talking about by propagating the perceived issue?
It's not WotC who is bringing up this issue, it's people such as yourself. You are reinforcing the sterotyping that you claim that WotC is reinforcing. If you would not have brought the issue up in this thread, there are probably a lot of gamers who would have never had it enter their radar.