In fiat economy the share of shadow economy is 15-40% and sometimes even more, I think thinking that only 2% of Bitcoin transactions are unlawful is quite naive. You don't need to sell drugs or weapons to make illegal transactions, simply working or running a business without a license already counts as unlawful, I believe that trading is the biggest use case of Bitcoin at the moment, but after that there's a huge amount of services that very likely belong to shadow economy.