Jay
If someone tells you they are going to buy drugs with bitcoin then you have a legal duty not to sell bitcoin to them. Just like you have a legal duty not to sell someone a pillow if they tell you they are going to use it to asphyxiate their mother in law. And no this is not a govt conspiracy against pillow dealers.
http://en.m.wikipedia.org/wiki/Accessory_(legal_term)
Thatt you may consider this to be an unreasonable intrusion of the government into your affairs is not particularly relevant.