I believe the responsibility for the harm caused by addictive drugs lies on the user to such great extent, that whatever remains for the people who facilitate the sale is not enough for it to be a criminal offence. It's still immoral the same way it's immoral to operate a gambling shop.

But in Ulbricht's case I'd say even this part is mitigated by the fact that facilitating the trade of dangerous drugs was a side effect of running a useful service for responsible drug users.