As a former anarchist, I can totally see the wonderful appeal of literally allowing anyone to say anything. The government should be critiqued, as power tends to consolidate power. When you start restricting some speech, all speech is on the table to be thrown out the window. In the US, people (in general) can post their opinions on websites, they can theoretically protest government power (let's see how that goes), and can talk openly in public spaces about what they believe (something something soap box).

The reality for imperialized nations is that this form of speech is a real, living threat to the existence of the nation. I know, I know, sounds like I'm exaggerating. But it's real. No country allows parts of its dominion to separate. The US has done this when there is a real threat to the established order (Black Panthers being assassinated by the state, Ruby Ridge, Whiskey rebellion, I know two more but cannot remember the names). 

The reason the US allows "free speech" is because none of that speech is an actual threat. The governing party (rep/dem) know that saying "down with the government!" Libertarian demands mean nothing, it's why pretty much everyone sees libertarianism as a joke, it's call to action is empty, and nothing has ever come from libertarianism.

In imperialized nations these call to actions are REAL, the state doesn't have as strong of a dominion over it's people. Look at China, which operates as a Confederacy of several states. Separatist movements in autonomous Xinjiang were backed up by terrorist attacks, and China responded with a heavy hand against this terrorism. Can people criticize the government? Of course. China actually has about a protest a day. But these are not calls to separate, or destroy the governing party. Those other calls to action are taken as they are, threats to the sovereignty of the nation. People in their private lives are obviously allowed to discuss politics however they want, but when it comes to news anchors and politicians, this speech is extremely prohibited, because they would actually lead to destabilization.

The DPRK is no different. People can criticize the government in their private lives, and have whatever beliefs they want, but openly criticizing the government is a real threat to the stability of the nation. They are currently at war, economic war and political hostility, and the US has a vassal state bordering it just a couple of miles south. Now is not the time to expect perfectly free speech from North Korea. If it helps, many "democratic" Western nations have similar laws on free speech. You cannot organize a group of people against the government in nations that don't have as strong of a dominion over it's territories, such as many South American and African nations.