I think you're right that the seeing vs. existing is a big part of why people's intuitions about salmon vs. Mohammed may differ in the example. British people (in the example) aren't trying to stop the existence of salmon pictures they can't see, whereas some Muslims are trying to stop the existence of Mohammed pictures they can't see. Even if only a minority of Muslims holds that attitude, it might be sufficiently annoying and scary to some non-Muslims that they are willing to annoy other Muslims by making pictures as a protest.

Yvain almost covers this case:

Say a random Christian kicked a Muslim in the face, and a few other Muslims got really angry, blew the whole thing out of proportion, and killed him and his entire family. This would be an inappropriately strong response, and certainly you could be upset about it, but the proper response wouldn't be to go kicking random Muslims in the face. They didn't do it, and they probably don't even approve. But drawing pictures of Mohammed offends many Muslims, not just the ones who send death threats.

Except kicking someone in the face violates Western notions of rights, while drawing pictures of Mohammed somewhere doesn't. Drawing pictures of Mohammed to protest Muslims who try to deny the right of others to do so is not a violation of anyone's rights, according to Western concepts of rights. Westerners feel they must treat any attempt to deny their rights as a Schelling Point.

Yes, it is still annoying for the set of Muslims-who-are-bothered-by-pictures-of-Mohammed-but-aren't-trying-to-take-away-the-right-of-people-to-do-so. Yet people who draw pictures of Mohammed might feel they are justified in annoying those Muslims in order to protest the subset of Muslims that attempt censorship. Perhaps they hope that moderate Muslims will understand why they must defend that Schelling Point, or perhaps they hope that moderate Muslims will bring the radicals in line.