An important distinction between the salmon and Mohammed pictures was omitted. Namely, the potential to change the offended in the long run.

In both examples the differences between two groups are potentially dangerous. There is a direct risk of accidentally showing a salmon picture in presence of a Briton. There is a graver risk that the salmon thing would cause further distrust between Britons and non-Britons, because it's somehow hard to not laugh at somebody who becomes enraged because of a picture of a salmon. There may be bombings of printers who publish fishing books, or any other kind of violence. The world is almost certainly better without such controversies. So it may be reasonable to try to eliminate them.

In the salmon example, the Britons have a physical reason (chips in their brains) for their salmon obsession and they can't be (presumably) taught to ignore the salmon pictures. On the other hand, Muslims' rage over depicting the prophet is a cultural habit amenable to change. People are usually less offended (in whatever meaning) by stuff which they encounter on a daily basis. Exposing the Muslims to Mohammed cartoons may significantly move their offense threshold towards more tolerance.