Consider us cynical, however I doubt he was arrested because of his threat against Darren Wilson. It is perhaps more likely this man was arrested because he represented a political risk to exposing the hidden undercurrent/narrative.  If his activity had not been stopped he could potentially have exposed the entire construct of the leftist’s Ferguson ideology based around racism and hatred. That exposure, and not the risk to Wilson, needed to be managed removed.
UPDATE: SUSPICIONS CONFIRMED:

[…]  “It is unclear how exactly authorities focused in on Abdul-Jabbaar, but the key FBI agent on the case is currently assigned to an international terrorism squad, part of the Joint Terrorism Task Force out of the the FBI’s Seattle Field Division. In addition to regular posts about the Ferguson shooting, Abdul-Jabbaar’s Facebook page includes many posts about the Islamic State (also known as ISIL or ISIS), and his cover photo is the Islamic State’s black flag.”  (link)

46 year-old Jaleel Tariq Abdul-Jabbaar starting posting threats to Facebook shortly after the August shooting of Michael Brown. Investigators said the threats escalated and continued through late November – story here.
Abdul-Jabbaar also threatened other officers and even attempted to use Facebook to buy a gun, police say.
Acting U.S. Attorney Annette Hayes said freedom of speech does not extend to threats to kill or injure law enforcement officers.
“Although we each have the right to express our views about the decision reached by the state grand jury in Ferguson, Missouri, we cannot tolerate violence or threats of violence that are intended to intimidate, and ultimately silence debate,” said Hayes in a written statement. “Such threats are crimes, and the individuals who make them must be held to account.”

If convicted on all three counts of making interstate threats, Abdul-Jabbaar could face up to five years in prison.  (link)
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