It is not at all one of the fundamental tenets of our political system. You have to take social context into consideration when generating social laws. People in oppressed and minority groups need more help because they are starting from a position of less privilege. And this is what we do: Laws like the Civil Rights Act of 1964 exist solely to protect the rights of people who are being oppressed and to restrict the rights of people doing the oppressing. Society acted to defend against intolerance, by being more intolerant of intolerant people.
The basic slippery slope you envision is "if we restrict rights, from what principle do we decide where to stop doing that?" The fault in the argument is that it is totally reasonable to not tolerate intolerance, and again, uncritical tolerance is not what we desire here, but rather tolerance of viewpoints which are not intolerant.
The basic slippery slope you envision is "if we restrict rights, from what principle do we decide where to stop doing that?" The fault in the argument is that it is totally reasonable to not tolerate intolerance, and again, uncritical tolerance is not what we desire here, but rather tolerance of viewpoints which are not intolerant.