If it's okay to arrest people making abstract threats of violence online because a small percentage of them actually do it, where's the flaw in taking it one step further and arresting every person who has ever played an FPS, because a small percentage of them actually do it.
What percentage of people reading this thread ever did any of the following when they were under the age of 12:
A) kissed/pinched/inappropriate snapped the clothing of a member of the opposite sex?
B) wrote down a reasonably detailed retribution story about what they wish would happen to at least one classmate?
C) brought a toy gun or a realistic looking replica weapon to school?
Well, kids that do any of these things in this generation are now being slapped with felony charges; meanwhile, if they do something wrong they actually CAN conceive the full depth of, such as shoplifting, they get their fingers slapped.
Priorities, people. Law enforcement should develop them. I can't believe any competent system of threat triage would have resulted in the incident in the OP ever coming to the forefront. Let's arrest every person who has a drug reference in their name in their handle while we're at it. As George Carlin said, "don't wimp out on me now! The blood is already on our hands, we're just talking a difference of degree at this point"