From that essay:

Agreeing to tribute costs the victim not only the tribute but the only available Schelling point.

This is where all the slippery-slope arguments come from. Without "speech free from repercussion" as a Schelling point, there appears to the West be no other natural point until you reach complete submission to Muslim dictates. The Muslim tradition would prefer if the conflict was resolved on their side of the issue - in itself a strong strategy for moving towards more complete submission to Muslim tradition. (Given this, an unreasonable attachment to what the West perceives as the Schelling point is a sound strategy for the West - cue cries of free speech).