It feels weird to e that people are defending an approach here: These are fantasy worlds, they have fantasy rules, and DMs can do whatever works for them and their group. Your goblins can be killing because it is the way they know how to get food to survive, because they have an innate instinct for evil, or because they have no souls. Whatever works for you - all of these options can be part of a great story.
I agree that a fantasy world can and should work differently. We don't have real goblins in our world so it's up to the DM to decide what they really are and whether they work as written. Maybe the DM is a big Harry Potter fan and goblins are mostly friendly if a bit ugly and sometimes misunderstood. Heck, in LFR campaign I played a gnoll. I have no problem with monstrous PCs per se.
However, I do think a campaign should have internal consistency that I personally find lacking in FR. If goblins are neutral evil black-hearted little humanoids motivated by greed and malice as stated in the MM, then I don't see them being accepted in most places unless that's just a general descriptor.
But if it's just a general descriptor then should there be a moral dilema if the wizard starts fireballing a goblin village? That question may add a flavor to your campaign that you want, but for others (including me) it's just an unnecessary complication to a game. In addition in FR there are times when monstrous creatures are just misunderstood and discriminated unfortunates and other times just monster fodder. There's no logical consistency.
Or does society for some reason accept that if a group of goblins find a vulnerable child in an alley that they will torture and kill them just for fun because that's what goblins do?*
I'd rather take the possibly over-simplistic approach and say that monstrous humanoids are evil and not allowed because I have plenty of twists, turns and moral dilemmas in my campaign without adding an unavoidable layer on top. If evil is clearly identifiable most of the time, it's more of a twist if someone you trusted turns out to be evil.
*Assuming that goblins are run according to the description in the MM and are not Harry Potter goblins of course.
P.S. I should note that this is a common problem with a lot of TV shows and films as well. The protagonists mow down dozens of low level guys without blinking an eye but then get to the big bad and they can't just kill this murderous sociopath because then they'd be "just like them". What? You just killed how many guys who may have just been there for a paycheck or conscripted and
now you grow a conscience?
