They don't want users with illegally obtained bitcoin to participate in coinjoins. One of the problems is that no one (not even Wasabi or zkSNACKs) knows what isn't allowed and why. They simply trust the decisions of their blockchain analysis partner. And don't bother asking them why they do it. If you do, their forum representative will tell you the software is open-source and that they make donations to the Human Rights Foundation and the TOR project. Seeing as you are wearing the signature of a mixing service, they will throw in a few sentences about you advertising a scam and/or that you are a thief or scammer yourself.