Chinese walls/firewalls don't quite hold up in court as well as one might think. Obviously it can depend on the country and circumstances, but anytime people feel there is a criminal conspiracy at work, they come up you hard. It is also hard to keep such a wall "clean" as all it takes is one person being stupid to establish a link.
Again, it's also about fear and human nature, not reality. I'm sure there are people with great opsec and planning who don't have this problem, but they also don't necessarily attract enough attention to get shut down. Factor in that many admins start a site because of ego, and that doesn't lead to great opsec. Nor do most people want to use software they know nothing about.
I should probably add FWIW, married to a prosecutor who goes after international criminal conspiracies among other things. Not the computer stuff thankfully, but I've seen and heard about what happens when the words criminal conspiracy get thrown about and people with money are involved. Doubly dangerous when extradition treaties exist or the country handling the case is a bigger power like the US.