In addition to the great replies already given, part of the reason also lies in the difference between authoritarianism and totalitarianism which I have described more in-depth elsewhere on this site.
From a totalitarian point of view, the ruling group wants to control the minds and ideas of the population, not just keep power. Therefore, dissent is outright dangerous as it shows the possibility of the government being wrong. On the other hand, as the dissident is disagreeing with the true teachings, they must be silenced before they even get a chance to rise to power.
In an authoritarian system, the underlying ideology is often not seen as important. Instead, the most important goal for the ruling group is to stay in power and alive. There is not necessarily any need to preach any specific ideology and so dissent in and of itself can be less of a problem. The defining factor changes from 'can this, even theoretically, become a danger to what I say?' to 'is this threatening me and my power right now?' If the government thinks that a threat is sufficiently contained with an opposition leader in prison, then why go any further?
In the real world, the distinction between authoritarian and totalitarian is not as strict as I have outlined it here theoretically. Regimes can and do have features of both systems and over time may develop from predominantly one to predominantly the other. Nevertheless, I can't help but notice that the examples you gave in the question tend to fall more into the authoritarian camp, Havel probably being the single exception. By contrast, one might cite Stalin and Kim Jong-un as examples of more totalitarian systems that did exterminate as many opposing forces as they needed to secure their power, especially in the early stages of their respective reigns.