Do you think the police are being ignored and not trusted in cases of online violence? Perhaps this is why some victims don't follow what I would imagine to be the police advice to not publicise the threats against them.
Unfortunately police work takes longer than the time remaining before the event. Better screening and more security is the obvious option, but apparently Utah allows people to carry regardless so it wouldn't do much good.
> This made me wonder: has there ever been a death threat, that has actually been carried out later by the same person or group?
ISIS does it with Westerners on a near daily basis these days. The IRA would generally phone in bomb threats in advance of bombs. I'm sure there are other examples.
> Why would anyone announce a premeditated murder before it really happens?
Intimidation. Do it once and the next time you threaten to kill someone if they don't do what you want they're a lot more likely to take you seriously.
Are you telling me I can send an email threatening mass murder to whomever I want and there are no consequences.
We've seen kids thrown in jail for indirect comments on Facebook, surely this is more tangible.