It is the perception of it being true that is truly damaging, regardless of the facts. As the article mentions at the end, it "Other Internet intermediaries should now expect to receive a phone call when some other member of Congress is unhappy with speech they are hosting. After all, it worked on Amazon."

If Amazon had just kept the site up, OR if there was no call to their offices about it, and they removed WikiLeaks on their own, the perception would be different. A more independent Amazon and a world where US companies can make their own legal decisions about content without Gov't interference or coercion.

Now the perception is someone as big as Amazon can get scared by a phone call, and no place is safe to put a site that might tick off a gov't official, and they get ticked off at a LOT of stuff.