The difference between the UFC and other orgs is in the billions.
Yes other promotions and fight orgs are just as corrupt (hell, even more in a lot of instances), it's just the UFC is the most successful of them. They are all predicated on taking as much advantage as possible of the fighters because that's where all the profit is.
So when the UFC does something like fuck over a guy that's literally put his life on the line for company repeatedly for over a decade and made them tens of millions for a measly quarter mill that he actually deserves, they just further reveal their true colors again as a heartless blood-sucking vampire squid that likes to operate mafioso style.
They didn't like that he wouldn't play their stupid game of being forced to fight a heavy opponent at a huge disadvantage for a minor financial gain (what's 30% of Pereira's purse, $30K?) or take a short-booked fight in a few weeks and have to cut weight again and fight against somebody that's probably dangerous in skills but has little to no name value.
They could've just paid him under the table and he wouldn't' have even mentioned it, he would've been like "they took care of me" if he was asked. But they wanted to make an example of him and this behavior to set a precedent and put fighters on notice, that's why this went down publicly.