Even when guilt is established, they still don’t report to the police. Hence, this has nothing to do with the 2 witness rule. Its all about them not wanting to let the authorities in what they perceive as being “their business”.
Yes, of course there’s an element of that.
But as in everything I think the main reason is to protect their image - not to the police, the public or judiciary - but to Jehovah’s witnesses themselves. Above everything else they don’t want Jehovah’s witnesses to see A. there are criminals among them and B. that consequently Jehovah’s witnesses are just like any other religion and just like any other group of people.
Loss of face to ordinary Jehovah’s witnesses is more important to them even than money, because essentially “The Brand” is money in the bank to them.