You are confusing a single thread of an argument with a complete argument it. The it isn't illegal argument is used in conjunction with other arguments (a quick glance at how much the article talks about the image shows that it's inclusion is encyclopedic).
Does the image need to be shown, given its nature which is offensive to many, many people?
Should Wikipedia, which claims to be an encyclopedia and reaches millions of people daily—many, if not _most_, of them school students—really be distributing images such as the one that prompted the IWF ban?
In this case yes.
Prove it :-).
—Thomas Larsen