There's a big difference between writing "someone should kill person X" on a random webforum, which seems to be what you're referring to, and personally sending people threats, which is what I assumed the thread was about. Neither scenario is really acceptable, but the latter scenario is something that shouldn't be allowed. Period.
For the record, I know people who have received personal death threats for voicing opinions on the internet. To put it simply, it wasn't very pleasant. You have no idea how serious these things threats are(to be fair, they're usually just to scare you), but that's not very comforting then and there. It really does scare the shit out of you when people tell you that "they're going to break into your house while you're sleeping, rape you, and then slit your throat".
First of all, too many deaths are sent over the internet for the court system to ever be able to handle them all (and the same for society, we can't throw everybody in jail). Threats can - and should - be dealt with by the appropriate people.
I have been on the internet for 15 years or so. I've received death threats too, and context really is everything in this regards. I've received both normal threats or death threats for corpse camping and ganking people in World of Warcraft. Is the police and courts the appropriate people to fix that issue? No, the Blizzard Game Masters are. Similarly i have also received death threats here on internet forums. Should i call the police again, or simply just ask the admins to ban the user in question?
And even if i involve the police, what if the perpetrator is in a different country? Should the two nations invest resources in a cross-country investigation (and potential extradition)?
The world isn't black and white. A basic principle in security is that resources are always limited, and if you misallocate resources, you only end up less secure, not more secure. Therefore, to get the government involved, you need to show probably cause that something is worth investigating and handling (i.e. that there is an actual threat).
While i agree with you that death threats on the internet aren't acceptable, i disagree with letting the law handle it except in extreme circumstances, and i disagree with comparing it with real world death threats. They are still different in nature and - most importantly - different in actual threat level. And finally, they are also much tougher to investigate for the purpose of criminal prosecution.
Also, if people feel genuinely threatened by a statement you have made -- which, as far as I know, is usually why people press charges when they have received death threats -- and you're claiming that "you were just joking", you've done something horribly wrong and should get the fuck off the internet.
You're just underlining the point i made: that it's pretty much impossible to tell sarcasm on the internet. And it's also pretty much impossible for outsiders to tell the actual context.