You mean they created this currency, so that criminals would use it because of it anonymity and other advantages and when they do, they'll get caught? So, why haven't they done it already, people are scamming left and right, some hackers were also extorting people with that virus (if you remember) that encrypted all the files, even the celebrity fappening was sold for bitcoins and I am sure there have been other crimes where criminals used bitcoins but I don't remember seeing anyone getting arrested. Sure some exchange guys did but they're not the hardcore criminals government is after.