Without wanting to get dragged into a parenting debate (those seldom go well on HN)...the problem we face is that adults should have the maturity to chose which content they consume where as children do need some degree of protection from some content. People will obviously disagree on exactly where that line should be drawn, which is fine, but few would argue that there shouldn't be a line in the first place.

The real crux of the distinction is that obscenity laws are about censoring artistic flourishes (nudity, graphic violence, bad language, etc) that many feel is unsuitable for children, rather than filtering news content that is subjectively deemed as "wrong" due to a political bias. If the US (or any other government for that matter) abused obscenity laws to filter content aimed at children to an extent that then pushes a political bias (for example) then such a censorship would shift from a "good thing" to a "bad thing".