It is healthy in a democratic society to have disagreements and counter viewpoints about the government it has elected. With a majority rule, there will always be those that act as a counterbalance to the majority view. That is in essence the heart of democracy, to be able to hear multiple arguments and decide which course is best.
However, when these disagreements and differing viewpoints escalates beyond just a difference in viewpoints and one side of the argument begins to attack very structure and purpose of a freely democratic government, those voices are no longer constructive. Those voices become a disease that seeks to weaken the government to give room for anarchy and tyrants to fester and grow. For the sake of the society, we can allow those who spreads this poison to go unchecked. It is one thing to disagree about government policies or how it's carried out, but when you begin to question the very need for government, those are words of treason.