This is, IMO, likely due to the fact that people can't be held accountable for their comments if nobody knows their true identity.
(I'm only a single data point but..) I'm more likely to comment on something the more I can use my real identity as a by-line. Demonstrating one's involvement or knowledge in a certain area can be valuable (think HN or Stack Overflow).
To me, it seems the sort of comments that would be encouraged by anonymity are those that are particularly controversial, negative, or that break confidences.
(I'm only a single data point but..) I'm more likely to comment on something the more I can use my real identity as a by-line. Demonstrating one's involvement or knowledge in a certain area can be valuable (think HN or Stack Overflow).
To me, it seems the sort of comments that would be encouraged by anonymity are those that are particularly controversial, negative, or that break confidences.