What? The difference is that redditors don't claim to be journalists, so why would anyone put any weight in their accusations? No one was lynched by redditors (or in danger of being lynched), so let's not call this vigilantism.
Anyone who thinks that Salah Barhoun's life wasn't in danger for a few hours there just hasn't thought things through. This country is littered with small people with guns who would love to assassinate the newest boogeyman.
In any case, no number of intermediaries would absolve "the subreddit" of responsibility if something went tragically wrong as result of their actions. Mob psychology and the diffusion of responsibility certainly makes people feel like it would.
Sunil Tripathi's name will forever be linked to the Boston Marathon bombing for absolutely no good reason whatsoever, causing irreparable damage to his memory and untold grief to those who knew him on top of the already crushing grief of his being missing (and perhaps, it now seems, dead).