Oh I think I understand your point better now.  The moral question is really about what to do with the boost money, rather than some pre existing obligation regarding the post

I think the analogy is missing an element, i.e. that the criminal sees you with a curtain, and then slips you some money not to hide them. 

But point taken.

billytheked

I don't think they're morally obliged at all.  For one, downzapping doesn't actually remove the content, it just makes it less visible.  It would be like witnessing a murder and saying you're obliged to stand there with a curtain to block peoples' view of it.