Not enough voice actor to tell em.  
Plus it’s background ponies, not supporting ponies

@Background Pony #6945 True.

@Ferrotter  

 
This ^
 
You gotta let kids solve their own problems, if adults step in every time there is conflict they will grown to be cowards and conflict averse.
 
Twilight and Applejack did the right thing, they stepped in to diffuse the situations after the fillies did their powerful statements and disarmed Diamond who would certainly keep on pressing.
 
Had Scoots and Sweetie not intervened and had Diamond pressed even harder than I guess it would be alright to put a hoof down, but it would have to come from a family member not a random adult.

because half of them probly work for him in some capacity, or owe him money, or are related to someone who does, ponyville is a small town and hes the richest pony in it…do the math

@bluemeganium  

 
Well, there’s someone hanging around here whom I’ve known for a dozen years already who’d disagree in Diamond Mint’s case, but where are the Orange Blossom or Tornado Bolt fans?

Shock and uncertainty how to deal with it. Do they merely say something and (as they know from their own foalhoods) likely make it worse? Actually discipline two other families’ foals? (One of them at her own Cuteceñera?)
 
Given enough time one of them may have spoken up, but Sweetie Belle and Scootaloo beat them to it.

Spoiled Milk

Because Adults Are Useless.

They’re not popular enough to be defiant.

Bystander effect.

…beeeecause they pay Diamond and Silver protection bits and don’t want to draw their ire?

@bluemeganium  
Ikr? It’s weird how background ponies are handled.

@TheMultiBrony21  
At least you’re caring for Minuette with some others. There are still many ponies waiting for some attention. These are the ones they still can’t open their mouth. Not that they’ll fall more into oblivion for saying “wrong” things.

@bluemeganium  
Yup. That’s for sure!

Many background ponies are mentally damaged from the lack of attention and social isolation.

They want to be Diamond Tiara’s friends.

Because they’re not story important.

I don’t know, maybe it’s like in real life and they’re just secretly recording this…