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there are certain news sources that will make a point of avoiding the fluffier stuff, whereas others will embrace it completely.
Those are editorial decisions and those decisions are made by people. Those people are motivated by different things. The idea that a definition of news, and thereby newsworthiness, is flawed because people are involved, instead of numbers, is pretty untenable.
You have made two points here
Just because loads of people want to see pictures of members of the Royal Family sitting on a bench doesn't make it any more newsworthy than an austerity march
And
Surely the sheer number suggests that the matter is being taken very seriously and continues to be newsworthy.
You can't use the weight of numbers both against and for your standard of newsworthiness.
Yes and no: there are certain news sources that will make a point of avoiding the fluffier stuff, whereas others will embrace it completely. The point you make about the differing perceptions in the US shows this to be true.