I guess we can agree on that. Part of their spreading can definitely be credited to Pascal's mugging. Other people might find them amusing (especially the original creators of those messages).

I also found different forms of Pascal's mugging in different internet media: There are similar chain messages in instant messengers which promise you additional smileys / finding the love of your life within the next 24 hours / other [promised benefits] if you spread them. I guess these [promised benefits]-posts' popularity can barely be attributed to people finding them amusing.

Of course, this is Pascal's mugging for irrational people who believe in Omega / god / superstition.

But in the [gruesome details]-messages it's not so very irrational, it depends on your utility function. Most people value themselves more than others, and rational egoists, having a utility function taking only their specific state of mind into account, could assign such a high negative utility to themselves being tortured and killed that it's rational for them to spread the message.