No, that may seem fact to you because of your limited understanding and ignorance how things actually work.

Witnesses are not the only ones who validate transactions, all full nodes do that. So, witnesses have no power to censor any transactions because that would go conflict with all the full nodes. All they do is prevent double-spends. If witnesses only witness the transactions and cannot change or influence the transactions any way then they can't be held responsible for those either. All they do is just witness that these transactions took place and they don't witness those that all full nodes identified as invalid.

I am pretty sure that your limited brain power hasn't even figured out yet why using blackbytes for illegal activity is bad idea because you don't actually understand the risks and how blackbytes actually work. Either way, it is not meant for illegal activity, it's for privacy. Privacy doesn't have to be illegal. Illegal markets is your wet dream and it is minuscule market compared to all the legal activities that could be done with cryptocurrencies.

So, the only those users go to jail who do the illegal stuff thinking that DLTs are good for that use case, witnesses or other full nodes are not one of them. Just like Silk Road, cryptocurrencies can give you somewhat privacy to do that, but eventually you fxck up anyways somehow, just matter of time.

So, use it for privacy, not for illegal activity.