I think the flip side to the nobody trusts anyone is once you out yourself as a hitman with a good reputation ... I think you can assume any customers will be cops pretty dang fast.
Aye. With hitmen there are bodies, a capital crime, and no statute of limitations. Drug laws are often much more lenient, not as far reaching, and some random guys Silk Road order is a drop in the bucket compared to the serious importing and production of said drugs.
Plus you order your goods to the wrong address or use a fake name, and when the heat comes around you say "it wasn't me" and flush your drugs -- they're not going to chase you for months the way they might a murder case.
Obviously exceptions for the guys ordering kilos of coke every quarter, but as long as you put a little bit of effort into CYA no one is going to kick down your door for $80 worth of Molly.