Because one of the hitmen he hired was a scammer, and another was an FBI agent. Still clearly a crime to hire them for murder.
Ulbricht's right-hand-man Roger Thomas Clark, who was involved in one of the murder-for-hire conversations, admitted the conversation was real during his trial:
"In his own remarks, Clark didn't comment on that murder-for-hire conversation—which he at one point claimed had been fabricated by Ulbricht but later conceded was real."