The Supreme Court has ruled that only specific and credible threats can be prosecuted. Other threats, distasteful and disgusting as they are, are protected as free speech. It's gonna be very hard to prove that an online troll mouthing off constitutes a specific and credible threat. And the Justice Department is usually not interested in borderline cases, both for efficiency and out of fear of creating unfavorable precedent.

This is a country where you're allowed to burn someone in effigy and call for their execution, protest soldiers' funerals and celebrate their death as punishment for America tolerating gays, call for the overthrow of the US government, hold up a mock severed-head of the President, publicly hope for his assassination, etc. We take a rather expansive and absolutist view of free speech.