Applying moral principles to the real world requires complex reasoned judgement. Making the principles pure or absolute is an attempt to make the required judgement formulaic instead, often due to a cynicism about individual judgement abilities of people, and this wittingly or unwittingly leads to a paradoxical outcome like in the paradox of tolerance.
The paradox of unlimited tolerance is that it has unacceptable consequences in allowing the destruction of a tolerant society; same for pacifism if the grounding principles for it are valuing human life and preferring positive-sum outcomes instead of just non-violence being an end in itself.
There obviously can be and are many people arguing for absolute principles (often in bad faith, since they don't actually hold the principles themselves), which is what makes it so topical.
Presuming justified exceptions to a principle to be impossible is what makes something an absolutism fallacy in the category of fallacies of presumption.
The closest existing label turns out to be absolutism fallacy; I've posted a more focused question about the same topic elsewhere.
You've set up a dichotomy between limited (e.g., reciprocal) tolerance and absolute tolerance by presuming that the limitations would be arbitrary and unjustified, but the limitations are justified by self-preservation (in case of Popper, tempered by preferring rational discourse if possible), so what you've said is an illustration of the fallacious argument this question is about.
Too bad it needs Java.
You can use a simple bookmarklet to make links on LW non-functional, e.g.:
To use this you would just set up a bookmark with the code as the address and click on it while LW is open.
Edit: A cross-site version of the bookmarklet would look like this: