Slacktivist wonders in this post what people are thinking when they continue to pass this rumour on. Can they be so stupid as to think it is true? If they don't think it's true, why are they passing it on? When they boycott the soap, based on an obvious lie, what are their motives?
Lot of opinions in the comments, but the one which seems most convincing to me is that stories like this express strong shared feelings. In this case perhaps feelings of fear and helplessness, that big business is malevolent, and that 'out there' is a threatening place. The stories reify and externalise these inchoate feelings, and when people act on them (boycott a product) that brings emotional relief, like a ritual.
This post links in my mind to this article in today's Guardian about the strange truthy/fakey stories that are circulating about Obama. The stories about the Large Hadron Collider seem similar too.
If Obama is not elected on the basis of this mad emotional storytelling it will be dreadful.