I think most decent people would be willing to go to some trouble to avoid taking pictures of salmon if British people politely asked this favor of them. If someone deliberately took lots of salmon photos and waved them in the Brits' faces, I think it would be fair to say ey isn't a nice person.

See, this is exactly where your analogy falls apart for me. The Muslims to whose behavior people are objecting in "everybody draw Muhammad" are not politely asking for the favor of avoiding creating images of Muhammad in future. They are approaching creators of existing images with serious threats of violence. In situations like the South Park incident, it seems quite distinct from the infliction of psychic pain in that - well, to blow another hole in the salmon analogy, does it mean that Americans should stop making television programs about how to cook salmon? So... yeah.