Because it is, or at least it's definitely political. Nobody seriously contends that by punishing people for actions like this you're actually preventing murders or terrorism in the real world, it's all about Setting An Example and Making It Clear We're Serious. (This does not, I hasten to add, mean that I think
harassment should be tolerated; but harassment goes significantly beyond making some kind of context-free 'threat'.)
This is indeed a stupid argument. Freedom of expression is trumped when it comes to matters of public safety in every western nation (I don't know how others handle it). The degree of such varies, but that's not the point. You can't shout fire in a crowded movie theater is a prime example of this public safety vs freemdom of expression. You have the right to not be silenced, or have views forced upon you by the federal government in the US. That does not give you carte Blanche to say whatever you want. Death threats fall into this category.
I absolutely agree that there are situations where freedom of expression needs to be restricted for the collective good, but the consequences of imposing those restrictions are serious enough that they need some pretty major utilitarian justification behind them.