im not buying or using drugs but i just want to share my thought about this. anyway, some people may use bitcoin in buying drugs or any illegal activities. this is because using bitcoin as a payment will not require you to divulge your real identity. thats is why some prefer to use bitcoin in dealing with illegal acts.
Of course because we are in illegal transaction, but you cannot hide that longer, if the government will exert more effort to go after criminals, they can find us even if we are using bitcoin. It's not bitcoin alone are being sure as monero is also popular in dark market.