True, the threat is more credible, but ... free speech, anyone? It's easy to defend free speech when it's the virgin Mary in a jar of piss, while the Catholics complain (but don't go and slaughter the artist). If the 1st amendment is only going to be protected when something is offensive to a peaceful group (or one without the credible likelihood of violent radicals going ape), then you end up catering to the sensibilities of those most likely to be prone to violence.