Except it does. Humans and hobbits are both explicitly described as having varying skin tones so the mutations for such exist in Middle-earth. After that it's about using that interpretation to fill in the gaps that need to be filled in order to adapt to the screen (namely, hiring actors). This isn't a world breaking, reality bending thing.
It always comes to the most absurd examples, huh? That in a world that does have examples of varying skin tones having a character whose skin tone isn't specified be anything other than white is on the same level as blue skin, lightsabers, and magical flight... You do see how seeing so much push back against something so natural in a setting where it already exists can become so fucking frustrating, right? I mean, this is the epitome of bad faith argument.
No rules are being broken. Humans (such as the first men of House Beor from whom the Numenoreans decended) and hobbits (Harfoots, who are the most common type of hobbit) exhibit darker skin than others of their race without massive deviations in culture or ethnicity. These rules are already in the setting.
Tolkien made an effort to distance himself from his early depiction of dwarves after the Hobbit (around the time when certain people in Germany were looking for any way to dehumanize Jews). The connections were more in language and the diaspora that guided the quest of Thorin and his band. Again, drawing the connection all the way to skin color is purely assumption on your part. Not to mention that being black and being Jewish aren't mutually exclusive. Are they more rare? Sure. But so were/are Jewish people with red hair, and I imagine until now you were perfectly fine with red haired dwarves.