You are using the terms "partnership" and "cooperation" to describe a relationship between a customer and a business, which is misleading. There are companies whose business model involves aggregating reports of coins being stolen, and zkSNACKs buys those reports in order to avoid accepting those stolen coins. If zkSNACKs were to buy a McDonald's hamburger, that does not mean "zkSNACKs is partnering with McDonald's".
Wasabi or zkSNACKs aren't private customers, though. They are businesses using the services of other business entities (blockchain analysis) whose version of the truth and estimation affects their own operations. In other words, you accept the decision of the blockchain analysis company's views regarding the cleanliness of my UTXOs. My participation in your coinjoins depends on the truth the blockchain analysis firm serves to you. I call that a partnership and cooperation. You are free to use any other terms you like.
How do you know stolen coins are getting rejected? You keep talking about open-source, where can we see these rules publicly?
According to whose criteria will certain UTXOs be added to whatever list is being used to determine naughtiness of coins?
Who made that list and is whose name? What entity or government agency is responsible to maintain it, make changes, and decide what's good and what's bad?
Who do I get in touch with if my UTXOs were rejected for reasons unknown to me?
Being in possession of stolen money or other means of payments doesn't necessarily make the owner a thief. You do understand that money and crypto circulate. As someone involved in the crypto business, you should know that those rules your blockchain partners present to you can't be applied to cash and fiat, otherwise a lot of it would have to be confiscated and taken out of circulation because many bills had contact or were used in illegal actions at one point in their past.