It seems to me that the promise of some great outcome seems either/both unbelievable and easy to defer action toward. But negative outcomes are immediate and demand action now. Negatively-charged stories seem to press all the same buttons -- "can you believe this!?", "this must stop!", "they're at it again!"
So it would seem to me that the more interesting question is why would they like being indignant so much? Does it foster some group cohesion or does it trigger our flight/fight response so that people get a dopamin rush?
So ironically, the "righteous individual against nefarious powers and / or the establishment" turns out to be a mechanism for preserving greater social cohesion.
The study (8.3mb PDF) is here: https://marketing.wharton.upenn.edu/files/?whdmsaction=publi...
The main takeaway: "How do specific emotions affect virality? The relationships between specific emotions and virality suggest that the role of emotion in transmission is more complex than mere valence alone (Table 4, Model 3). While more awe-inspiring (a positive emotion) content is more viral and sadness inducing (a negative emotion) content is less viral, some negative emotions are positively associated with virality. More anxiety- and anger-inducing stories are both more likely to make the most e-mailed list. This suggests that transmission is about more than simply sharing positive things and avoiding sharing negative ones. Consistent with our theorizing, content that evokes high-arousal emotions (i.e., awe, anger, and anxiety), regardless of their valence, is more viral."
Humans are confident, fighty creatures who like to beat the odds. Or at least, they like to think of themselves in this way.