They need third parties to think that working with them is their best option. Their best option is to do something legal, but desperate people can be manipulated. Greedy people can be really manipulated. Maintaining the illusion of safety is just marketing.
If the game is extortion, then psychological manipulation is part of the game. I have no assurances that you've given me $1.4 million today and this is the last I will ever hear from you. If I know little about you I can't even turn State's Evidence to save my ass if I get caught but you don't. If I think I know that you or anyone with details of your operation won't come back later and threaten to turn me in unless I pay you money, then I'm a fucking idiot. You could ask for money, or to "deliver a package" for you. In that situation, what you and your associates are counting on me forgetting is that I am mortgaging my future by piling on new crimes with later statute of limitations dates, and establishing a pattern of behavior that makes me look even more complicit.
There is no honor among thieves. Stop trying to figure out how to pet a wolf without getting bit.
Why is a ransomware company going to follow the rules of common decorum when dealing with a collaborator if the whole point is to defraud a third party?
How do I know that they made the payout? Is the collaborator going to come forward and identify themselves? The only way you know that person got paid is if they don't get arrested. There are whole branches of organized crime than live and breathe in the space between 'got paid and lived happily ever after' and 'ended up on the front page of the NYT'. Just off the top of my head you have blackmail, extortion, and hiding the body.